Method and apparatus for retrieving information recorded on rewritable magneto-optical media

Method and apparatus for retrieving the information recorded on the rewritable magneto.sub.-- optical media is disclosed. According to the present invention, while a regeneration beam in synchronization with a clock signal is irradiated on a mark recorded on a rewritable magneto.sub.-- optical media, a regeneration signal detector detects a regenerated electrical signal from the recorded mark. On the other hand, a phase difference detector detects the phase difference between the clock signal and the regenerated electrical signal. The clock signal is delayed as much as the said phase difference and then its delayed clock signal is used as a sampling signal. The regenerated electrical signal is sampled and held in response to said sampling signal and then the regenerated electrical signal which is sampled and held by said sample-and-holder is converted into a regenerated bit signal.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
The present invention relates to method and apparatus for retrieving the 
information recorded on a rewritable magneto.sub.-- optical media, more 
particularly, to the method and apparatus for retrieving the information 
recorded on the rewritable magneto.sub.-- optical media having a function 
in which the clock signal to be used as a sampling signal is compensated 
in response to the phase difference between the clock signal and the 
regenerated electrical signal from the optical disk. 
2. Discussion of the Related Art 
A rewritable magneto.sub.-- optical media has been in practical use as an 
information recording media having a high density rewritable capability. 
In particular, the rewritable magneto.sub.-- optical media having a 
recording layer made of the amorphous alloy of rare-earth and transition 
metals have a remarkable characteristic. 
The principle of the rewritable magneto.sub.-- optical media is briefly 
explained by taking an example. By focusing a laser beam on the surface of 
a rewritable magneto.sub.-- optical media as a spot whose diameter is as 
short as the wavelength of the light, the temperature of the spot on the 
recording layer is raised up to 150.degree. C.-200.degree. C. If the 
temperature of the recording media heated by the laser beam goes up to 
Curie temperature (Tc), the magnetization of the spot is disappeared. At 
this time, if constant magnetic bias field is applied in one direction, 
magnetic mark (it is called a pit) is recorded on the recording layer by 
the magnetic inversion occurring when the heated area returns to the room 
temperature. 
Referring to FIGS. 1 and 2, the process for recording the information on 
the rewritable magneto-optical media is explained. FIG. 1 is a diagram 
showing a conventional type of recording apparatus, and FIG. 2 is a timing 
diagram to explain the operation of the apparatus in FIG. 1. Based on the 
information initially preformatted on the optical disk, a channel clock 
signal generator 9 generates a channel clock signal 10 shown in FIG. 2(a). 
In response to the channel clock signal 10, laser driver 11 makes the 
laser diode 1 emit pulse beam. Laser pulse beam 2 of FIG. 2 is irradiated 
on the optical disk 8 as a optical spot 4 through an objective lens 3. On 
the other hand, a magnetic head 5 which is closely disposed to optical 
disk 8 and is driven by signal generator 6 forms a modulation magnetic 
field 7 shown in FIG. 2(d). Therefore, a mark of FIG. 2(e), corresponding 
to a channel bit shown in FIG. 2(b), is recorded on the optical disk 8. As 
shown in FIGS. 2(a)-2(e), if the frequency of channel clock signal 10 is 
increased and the laser beam focussed as a spot 4 is irradiated on the 
optical disk 8 as a pulse, by the combination of the pulse type laser beam 
and the modulation magnetic field, the optical spot 4 in synchronization 
with the channel clock signal 10 is irradiated on the optical disk 8. 
Marks are overlapped and then recorded on the optical disk 8 by the 
optical spots irradiated like this. According to this method, magnetic pit 
which has the mark length shorter than the optical spot 4 is recorded. 
This method is a known technique published in Japanese patent publication 
Pyungsung No 1-292603. 
As a method for retrieving the recorded information from the optical disk, 
on the other hand, there is a method to focus laser beam with a constant 
output power as a spot whose diameter is as short as its wavelength and 
then irradiate as a spot on the surface of the rewritable magneto.sub.-- 
optical media. 
The focussed optical spot is reflected from the surface of the rewritable 
magneto.sub.-- optical media. At this time, the polarization state of the 
laser beam is changed by Kerr effect. By optically detecting the change of 
the polarization state of the reflected beam, the information recorded on 
the magneto-optical media in magnetic state is read from the media. 
However, as shown in FIG. 3, as the information is recorded on the 
rewritable magneto.sub.-- optical media in high density, the length of the 
magnetic mark is getting shorter and the optical spot is getting longer 
than the magnetic pit (or mark). As the result, a problem arises in the 
resolution capability when the mark is read out. 
In order to solve this problem, super resolution techniques have been 
attempted. As one of the techniques, a method of magnetically induced 
super resolution (MSR) using an exchange coupling force has been 
introduced. A method of using an in-plane magnetic layer which is a kind 
of the MSR is shown in FIG. 4. As shown in FIG. 4, the rewritable 
magneto.sub.-- optical media consists of two layers having an exchange 
coupling structure between a readout layer with a relatively low 
coercivity and a recording layer with a relatively high coercivity. The 
readout layer has the in-plane magnetic layer. However, when the 
temperature of the layer is over a specific temperature, the readout layer 
changes its magnetic orientation and has a perpendicular magnetization. 
The recording layer is a perpendicular magnetization layer so as to 
preserve the information. If the laser beam is irradiated on the readout 
layer in order to retrieve the information, the magnetization of the 
readout layer of high temperature area in the middle of the optical spot 
(the area with the temperature above threshold value in FIG. 4) is changed 
from the in-plane magnetization to the perpendicular magnetization, and 
then a polar Kerr effect comes out. In other words, the magnetic field in 
the high temperature area of the readout layer is changed into the 
direction of the magnetic field of the recording layer. On the contrary, 
because the Kerr effect does not occur in the low temperature area in the 
neighborhood, the magnetization of the recording layer is masked. 
Therefore, if the power of the regeneration laser beam is properly 
selected, the recorded information is retrieved from the high temperature 
area corresponding to the middle of the laser spot, and, as the result, 
the retrieving operation in super resolution is possible. However, by the 
reason that this method of retrieving a small magnetic pit (or mark) by 
masking the readout layer like this uses a subtle temperature distribution 
in the beam spot, the change in the magnetic orientation is affected by 
the fluctuation of the rotation speed of the laser disk and the change in 
the power of the regeneration laser beam and therefore is unsatisfactory 
As the result, a good carrier-to-noise ratio is not obtained. As the 
result, the error rate becomes high and the jitter occurs, and a good 
quality of the readout signal is not obtained. 
As a method to solve this problem, the technique irradiating a regeneration 
laser beam in a pulse synchronized with the channel clock signal is 
disclosed in Japanese patent publication Pyungsung 4-325948. According to 
this technique, there is a merit making the error rate very low. 
FIG. 5 shows an example of the apparatus for retrieving the recorded 
information from the rewritable optical media. Based on the regeneration 
clock signal of clock generator 58, a pulse shaper 57 outputs a pulse type 
of signal. In response to this pulse type of signal, laser driver 56 
drives the laser diode 55. The laser beam emitted in the pulse type from 
the laser diode 55 is focussed on the surface of the rewritable optical 
media 51 by collimator lens 54 and objective lens 52. The laser beam spot 
focussed on the media 51 is reflected from the readout layer and passed 
through the objective lens 52, and then comes toward the first polarized 
beam splitter 53. The optical spot is again applied to the second 
polarized beam splitter 59 by the first polarized beam splitter 53. In 
this splitter 59, p-polarization component of the beam spot is transmitted 
through the splitter 59 and s-polarization component is reflected from the 
splitter 59. 
The p-polarization component and the s-polarization component are focussed 
and then converted into electrical signals by the first photo detector 61 
and the second photo detector 60, respectively. The photoelectric 
converted electrical signals is applied to the difference amplifier 62. 
After the signals are amplified, they are applied to the regenerated bit 
stream detector 63. The regenerated bit stream detector 63 processes the 
output signal of the difference amplifier 62 and then generates a bit 
signal corresponding to the recorded information, that is, a binary 
signal. Usually, the regenerated bit stream detector 63 filters the output 
pulse signal of the difference amplifier 62 by using a lowpass filter and 
generates a regenerated bit signal of 0 or 1 by zero.sub.-- crossing the 
filtered signal. However, this conventional technique has the following 
weakness. As described above in detail, in the regeneration method 
irradiating the pulse type of laser beam on the optical disk, the 
electrical signal detected by the photo pickup is of pulse type. However, 
in the high density recording media as shown in FIG. 3, the detected 
electrical signal is too small as compared to size of the pulsed laser 
spot and is therefore easily corrupted by the noise caused by the laser 
beam pulse. Therefore, the signal-to-noise ratio is so bad. In FIG. 5, 
because the s-polarization component and the p-polarization component are 
very small, the output pulse signal obtained from the difference amplifier 
62 is also small. Moreover, the difference between the magnitude of the 
pulse signal corresponding to high signal 1 and the magnitude of the pulse 
signal corresponding to low signal 0 is too small. Therefore, it is very 
difficult to exactly determine 0 or 1 of the regenerated bit signal in the 
regenerated bit stream detector 63. 
As a method to overcome this problem, the method for retrieving the 
regenerated bit signal without any relationship to the s-polarization and 
the p-polarization is disclosed in Japanese patent publication So 
63-173252. According to this method, as shown in FIGS. 6 and 7, in 
synchronization with the regenerated clock signal generated at the 
regeneration clock generator 70, a pulse shaper 71 outputs a pulse signal. 
Based on this pulse signal, a regeneration beam generator 72 irradiates 
the pulse type of regeneration beam on the recorded mark of the optical 
disk 73. At this time, a signal detector 74 detects the electrical signal 
from the optical disk 73. Sample-and-holder 76 samples and holds the 
electrical signal in response to a sampling signal. A regenerated bit 
stream detector 77 obtains the regenerated bit signal from the output 
signal of the sample-and-holder 76. However, according to the structure in 
FIG. 6, the time delay from the irradiation of the regeneration beam to 
the detection of the electrical signal is not considered. Therefore, if 
the delay time occurs, the detected electrical signal is not sampled on 
the sampling instant and the right regenerated bit signal is not obtained. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is directed to a method and an apparatus 
for retrieving, without error, the recorded information from a rewritable 
optical media that substantially obviates one or more of the problems due 
to limitations and disadvantages of the related art. 
Another object of the present invention is to provide a method and an 
apparatus exactly detecting the regenerated bit signal from the rewritable 
optical media when the pulse type of regeneration beam is irradiated on 
the rewritable optical media. 
Additional features and advantages of the invention will be set forth in 
the description which follows, and in part will be apparent from the 
description, or may be learned by practice of the invention. The 
objectives and other advantages of the invention will be realized and 
attained by the structure particularly pointed out in the written 
description and claims hereof as well as the appended drawings. 
To achieve these and other advantages and in accordance with the purpose of 
the present invention, as embodied and broadly described, the method and 
the apparatus according to the present invention includes a 
sample-and-holder to exactly sample and hold the regenerated electrical 
signal detected from rewritable optical media under consideration of the 
delay time occurring when the regenerated electrical signal is detected by 
irradiating the pulse type of the regeneration beam on the rewritable 
optical media. 
Therefore, according to the method and the apparatus of the present 
invention, by irradiating a regeneration beam spot on the recorded mark of 
the rewritable optical media in synchronization with the clock signal, the 
regenerated electrical signal is retrieved from the rewritable optical 
media. Furthermore, the phase difference between the regenerated 
electrical signal and the regeneration clock signal is detected, and then 
the clock signal is delayed as much as the phase difference. The detected 
regenerated electrical signal is, in synchronization with the delayed 
clock signal, sampled and held. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory and are 
intended to provide further explanation of the invention as claimed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
Referring to FIGS. 8 and 9, the method and the apparatus of the present 
invention is described in detail. 
At first, the process for retrieving a regeneration electrical signal from 
the optical disk is the same as the conventional regeneraton device shown 
in FIGS. 5 and 6. The present invention is to provide the method for 
exactly detecting a regenerated bit signal from the detected regeneration 
electrical signal. In FIG. 8, if a regeneration clock pulse generator 100 
generates the regeneration clock signal of FIG. 9(a), a pulse shaper 200 
generates a pulse signal at the falling edge of the regeneration clock 
signal, in base on the regeneration clock signal, that is, in 
synchronization with the regeneration clock signal. 
If the pulse is applied to a laser beam generator 300 from the pulse shaper 
200, the laser beam generator 300 generates, based on the pulse signal, a 
laser beam of pulse type of FIG. 9(b) at the falling edge of the clock 
signal. In this apparatus, a laser diode is used in said laser beam 
generator 300. On the other hand, there is a lot of information recorded 
on the optical disk 400 as mark, as explained in FIGS. 1 and 2. The pulse 
type of laser beam is irradiated on a recorded mark of the optical disk 
400 shown in FIG. 9.COPYRGT.as a spot. When the irradiated beam is 
reflected from the recorded mark, a regeneration signal detector 500 
detects the regeneration electrical signal shown in FIG. 9(d) from the 
recorded mark. The detailed process in which the laser beam is irradiated 
on the optical disk 400 and the regeneration signal detector 500 detects 
the regenerated electrical signal from the reflected laser beam is the 
same as the explanation of FIG. 5. 
On the other hand, the detected regeneration electrical signal includes a 
reference signal to generate the clock signal of the regeneration 
apparatus. This reference signal indicates the synchronization signal or 
prepit which is included in wobbling signal. Therefore, when the 
regeneration electrical signal is detected, the reference signal detector 
1000 detects the reference signal from the regeneration electrical signal. 
And, the regeneration clock generator 100 bases on the reference signal 
and then generates the regeneration clock signal 
The reference signal detection like this is done from the early stage of 
operation of the regeneration device. FIG. 8(b) is a detailed block 
diagram of the regeneration clock generator 100, which is based on the 
reference signal included in the regeneration electrical signal and 
generates the regenerated clock. 
Referring to FIG. 8(b), when said reference signal is detected, a slicer 
101 of the regeneration clock generator 100 slices said reference signal 
and outputs the sliced signal, and the edge detector 102 detects the edge 
of the sliced signal obtained from the slicer 101. A phase difference 
detector 103 detects the phase difference between the output signal of the 
edge detector 102 and the output signal of divider 106. 
Lowpass filter 104 filters the phase difference signal obtained from the 
phase difference detector 103 and applies the filtered signal to voltage 
controlled oscillator 105. Said voltage controlled oscillator 105, based 
on the output signal of the lowpass filter 104, outputs an oscillation 
signal as the regenerated clock signal. Divider 106 divides one cycle of 
the oscillation signal into M equal parts and applies one of the M parts 
to the phase difference detector 103. In this embodiment of the present 
invention, a regeneration clock signal is generated in the base on the 
reference signal detected from optical disk 400. But this regeneration 
clock signal may be generated from the regeneration clock generator 100, 
without any relationship with the reference signal. On the other hand, the 
detected regeneration electrical signal is applied to the 
sample-and-holder 800 and the phase difference detector 600, and the pulse 
signal obtained from pulse shaper 200 is applied to the phase difference 
detector 600. The phase difference detector 600 detects the phase 
difference between the regeneration electrical signal and the pulse signal 
and generates the delay signal of FIG. 9(e) corresponding to the phase 
difference. When the delay signal is applied from said phase difference 
detector 600 to a pulse delay circuit 700, the pulse delay circuit 700 
delays the pulse signal received from the pulse shaper 200 as much as the 
delay signal and then supplies the sample-and-holder 800 with the delayed 
pulse signal as a sampling signal of FIG. 9(f). When the sampling signal 
is applied from said pulse delay circuit to the sample-and-holder 800, the 
sample-and-holder 800 samples the regenerated electrical signal received 
from said regenerated signal detector 500 and holds the sampled signal, 
and then outputs the sampled signal. 
When a regenerated bit stream detector 900 is fed with the output signal of 
said sample-and-holder 800, the regenerated bit stream detector 800 
detects a regenerated bit signal from the output signal of the sample-and- 
holder 800. In this embodiment of the present invention, the regenerated 
bit stream detector 900 consists of an adder 901 which adds the output 
signal from sample-and-holder 800 and the output signal from the lowpass 
filter 903, a comparator 902 which obtains a regenerated bit signal by 
comparing the output signal of the adder 901 with the reference signal and 
a lowpass filter 903 which filters the regenerated bit signal obtained 
from the comparator 902 and supplies the adder 901 with the filtered 
signal. Therefore, said comparator 902 substantially operates as a level 
slicer. 
According to the present invention, by sampling and holding the regenerated 
electrical signal and then by zero-crossing the sampled signal through a 
regenerated bit stream detector 900, the regenerated bit stream is 
obtained. Furthermore, by properly modulating the regenerated bit stream, 
the information recorded on the optical disk is read out. Therefore, even 
though the difference between the regenerated electrical signal 
corresponding to high level binary signal and the regenerated electrical 
signal corresponding to low level binary signal is very small, the 
regenerated bit signal is detected a little more exactly. 
Furthermore, when the regenerated clock signal or the pulse signal is used 
as a sampling signal for the sample-and-holder, by delaying said pulse 
signal or the regenerated clock signal as much as the phase difference 
between said regenerated electrical signal and said pulse signal (or clock 
signal) and by using the delayed signal as said sampling signal, the 
regenerated bit stream is a little more exactly obtained from said 
regenerated electrical signal. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in method and apparatus for retrieving the 
information recorded on a rewritable optical media of the present 
invention without departing from the spirit or scope of the invention. 
Thus, it is intended that the present invention covers the modifications 
and variations of this invention provided they come within the scope of 
the appended claims and their equivalents.