Information recording apparatus for recording/reproducing information by irradiating an information recording medium with an energy beam

In an information recording apparatus according to this invention, since it is not necessary to dispose any differential circuit, which was required heretofore, it is possible to fabricate it with a low cost and its error rate is also very small. Furthermore, since the mean reflectivity on the recording track does almost not vary by the fact that information is recorded there, no tracking off-set or focusing off-set is produced. Thus effects of this invention are remarkable.

BACKGROUND OF THE INVENTION 
This invention relates to an information recording apparatus for recording 
information by irradiating an information recording medium with an energy 
beam. 
As apparatuses for recording digital signals such as signals obtained by 
frequency-modulating analogue signals of image, sound, etc., data of 
electronic computers, facsimile signals, digital audio signals, etc. in 
real time, there are used apparatuses for recording information by 
irradiating an information recording medium with an energy beam such as a 
laser beam, electron beam, etc. 
Heretofore, as apparatuses for recording information by irradiating the 
information recording medium with a laser light beam, there are known 
apparatuses, in which various changes such as deformation, phase change, 
chemical change, change in the magnetic field, etc. are produced in the 
information recording medium, depending on the waveform of the laser light 
pulse. Among them those using deformation are disclosed e.g. in U.S. Pat. 
No. 4,238,803 and those using phase change are described in U.S. Pat. No. 
3,530,441. 
However, in such prior art information recording apparatuses, in the case 
where pulses of an energy beam, as indicated in FIG. 1(a), are projected 
thereon, the waveform of reproduced signals is deformed as indicated in 
FIG. 1(b) and it is not possible to specify the recorded position with a 
high precision. For this reason the reproduced signal is differentiated so 
as to transform it into a signal as indicated in FIG. 1(c) and the 
recorded position is defined as the position, where the signal level is 0. 
Therefore the prior art information recording apparatus required a 
differential circuit, which gave rise to problems that the signal to noise 
ratio decreased and that the apparatus was too expensive. Further there 
was another problem that tracking off-set or focusing off-set was easily 
produced, because the mean reflectivity on the recording track was varied 
by the fact that information was recorded there. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide an information recording medium, 
which is inexpensive and nevertheless whose error rate is small, in order 
to resolve the problems mentioned above. 
This object is achieved in an apparatus according to this invention, in 
which information is recorded by irradiating an information recording 
medium with an energy beam, by selecting a combination of the time (.tau.) 
necessary for the phase change in the recording medium, the amount of 
thermal diffusion (a) of the recording medium including a recording film 
and a protective layer, the radius (r) of the projected energy beam, the 
linear velocity (v) of the recording medium, and the cooling time 
(.alpha.) after having turned-off the pulse of the energy beam, so that 
after the reproduced signal obtained from a portion, where the variation 
rate of the energy given to the information recording medium with respect 
to time is large, moves in one direction, it moves in the opposite 
direction. That is, for a power-modulated pattern of the energy beam 
indicated in FIG. 2(a), reconstructed signals as indicated in FIG. 
2(c)-(h) or their up and down inverted signals are obtained, while 
heretofore it was reconstructed signals as indicated in FIG. 2(b) that 
were obtained. 
A more concrete example of the method is as follows. For example, in a 
recording medium, in which recording and erasing are effected by 
reversible phase change between crystal and amorphous by means of 
irradiation with an energy beam such as a laser light beam (light 
recording medium by phase change), the object of this invention is 
achieved by determining the time, which is necessary for the phase change 
from amorphous to crystal during the irradiation with a laser light beam, 
i.e. the crystallization time (.tau.) so that 
##EQU1## 
is satisfied, where .alpha.: time during which the temperature of a 
portion of the recording medium where the falling part of the energy beam 
pulse is irradiated passes through a phase-change temperature area, 
.beta.: time during which the temperature of a portion of the recording 
medium where the continuous part of the energy beam pulse is irradiated 
passes through the phase-change temperature area, 
a: amount of thermal diffusion of the recording medium including a 
recording film (3) and a protective layer (4), 
A: constant, B: constant, A&lt;B 
r: radius of the projected energy beam, 
v: linear velocity of the recording medium, and 
l: pulse width of the beam pulse. 
Moreover the amount of thermal diffusion a is represented by the following 
formula: 
##EQU2## 
For example, in the case where the composition of the recording film is 
Ge.sub.43 Te.sub.47 Se.sub.10 and the protective layer is made of 
SiO.sub.2, when .tau.=40 ns, a=0.5 (.mu.m).sup.3 /.mu.s, r=0.8 .mu.m and 
.alpha.=20 ns, the condition represented by Eq. (1) is satisfied and thus 
the recording method according to this invention can be realized, if v&lt;10 
m/s. 
In the case where the composition of the recording film is Ge.sub.43 
Te.sub.47 Tl.sub.10 and the protective layer is made of SiO.sub.2 on the 
light incidence side and Al on the side opposite thereto, when .tau.=10 
ns, a=0.15 (.mu.m).sup.3 /.mu.s, r=0.8 .mu.m and .alpha.=4 ns, the 
condition represented by Eq. (1) is satisfied and thus the recording 
method according to this invention can be realized, if v&lt;12 m/s. 
Further, when the diameter of the recording medium is 13 cm and the number 
of rotation per unit time is 1200 rpm, the linear velocity of the 
outermost periphery of the recording medium is 8 m/s. In this case, when 
the composition of the recording film is Ge.sub.38 Te.sub.42 Se.sub.20 ; 
the protective layer is made of ZrO.sub.2 ; .tau.=100 ns; a=1.0 
(.mu.m).sup.3 /.mu.s; and .alpha.=50 ns, the condition represented by Eq. 
(1) is satisfied and thus the recording method according to this invention 
can be realized. 
In this information recording apparatus, in the case where the reproduced 
signal is emitted only from a position, where the variation rate of the 
energy given to the information recording medium with respect to time is 
large, and where a light recording medium by phase change, whose 
composition is Ge.sub.43 Te.sub.47 Se.sub.10, is irradiated with a 
rectangular light pulse at a linear velocity of 8 m/s, in the rising and 
falling portions of the pulse, since the cooling time is shorter than the 
time necessary for the crystallization, the recording medium is made 
amorphous and in the continuous light irradiation portion of the pulse, 
since the cooling time is longer than the time necessary for the 
crystallization, the recording medium is recrystallized. Therefore, peaks 
of the reconstructed signal appear only at the rising and falling portions 
of the pulse. 
According to this invention, increase in the error rate due to jitter of 
the detected signal is small with respect to that obtained according to 
the prior art method, i.e. method, by which a signal, whose shape is 
similar to that of the given light pulse, is obtained and its rising and 
falling portions are detected (so-called pit-edge detection method). 
In addition, according to this invention, since the recording medium is 
made amorphous only at the rising and falling portions of the pulse and 
spike-shaped reconstructed signals are obtained only there, the mean 
reflectivity on the recording track hardly varies compared with the prior 
art method, which gives rise to no tracking off-set or off-set of the 
autofocus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention will be explained more in detail, using Embodiments 1 and 2. 
Embodiment 1 
FIG. 3 is a cross-sectional view of a part of information recording medium 
used in an information recording apparatus according to this invention. In 
the figure, reference numeral 1 is a base plate consisting of a disk made 
of chemically reinforced glass replicated with a groove for tracking, 1.1 
mm thick, whose diameter is 130 mm, which is made of an ultra-violet ray 
hardening resin disposed on the surface of the disk. 2 is a protective 
layer made of SiO.sub.2 formed on the base plate 1 by the 
magnetron-sputtering method, the protective layer being 110 nm thick. 3 is 
a thin recording film formed on the protective layer 2, the recording film 
being formed by the evaporation method by vaporizing Ge, Te and Se 
independently. 4 is a protective layer made of SiO.sub.2 formed on the 
recording film 3 by the magnetron-sputtering method, this protective layer 
4 being 110 nm thick. 
When such an information recording medium is continuously irradiated with a 
laser diode light beam of 14.1 mW, the irradiated portion of the recording 
film 3 is melted so that the elements in the recording film 3 react 
sufficiently on each other and that it can be locally crystallized during 
cooling after the irradiation. Now, when this information recording medium 
is irradiated with a rectangular laser light pulse for recording as 
indicated in FIG. 4A, a recorded portion 5 indicated in FIG. 4B is formed 
in the recording film 3. In this case, when the recording film 3 at the 
part 5A in the recorded portion 5 corresponding to the rising edge of the 
light pulse cools down, since the part adjacent on the left side to the 
irradiated portion 5 in FIG. 4B is not irradiated with the laser light, it 
cools down relatively rapidly, as indicated by a curve A in FIG. 4C, after 
the laser light has passed therethrough. Further, when the recording film 
3 at the part 5B corresponding to the middle part of the light pulse cools 
down, since there exists the laser light at the part 5B adjacent on the 
right side thereto and the temperature of the recording film 3 at the part 
adjacent on the left side thereto is high, it cools very slowly, as 
indicated by a curve B in FIG. 4C, after the laser light has passed 
therethrough. Still further, when the recording film 3 at the part 5C in 
the recorded portion 5 corresponding to the falling edge of the light 
pulse, since there exist no light beam at the part adjacent on the right 
side thereto, it cools down very rapidly, as indicated by a curve C in 
FIG. 4C. In this way, at the parts 5A and 5C, since the recording film 3 
cools down rapidly after the temperature thereof exceeds its melting point 
at least at a part of the irradiated portion, it becomes amorphous. 
However, since the cooling speed of the recording film 3 at the part 5C is 
greater than that at the part 5A, the proportion of the part of the 
recording film 3 which is amorphous is greater at the part 5C than at the 
part 5A. On the other hand, at the part 5B, since the recording film 3 
cools down slowly after the temperature thereof has exceeded its melting 
point, the recording film 3 is recrystallized and it becomes crystal. If 
the power of the laser light is set so that the highest temperature, which 
the recording film reaches during the irradiation, is slightly over its 
melting point, the temperature at the part 5A is slightly under its 
melting point and thus at this part the recording film is crystallized 
without melting. Consequently the crystallized state at the part 5A is 
somewhat different from that at the part 5B. This difference of these 
states can be detected also optically. Since the recording film 3 is thin, 
when the recording film 3 is irradiated with reading (or reproducing) 
light, it is reflected at the front and back side surfaces of the 
recording film 3. Since these reflected light beams interfere with each 
other, there is a minimum in the variation of the reflectivity with 
respect to the wavelength of the reading light, as indicated in FIGS. 5A, 
5B and 5C. In addition, since the reflectivity of the recording film 3 is 
smaller in the amorphous state than in the crystallized state, the 
wavelength of the reading light, for which the reflectivity is minimum, is 
shorter when the recording film 3 is amorphous than when it is 
crystallized. As indicated in FIGS. 5A, 5B and 5C, the relation between 
the wavelength of the reproduction light and the reflectivity of the 
information recording medium at the parts 5A, 5B and 5C can be represented 
by curves A, B and C, respectively. Furthermore, the wavelength of the 
reading light, for which the reflectivity is minimum, becomes longer with 
increasing thickness of the recording film 3. For this reason, it is 
possible to vary the wavelength of the reading light, for which the 
reflectivity is minimum, at the parts 5A, 5B and 5C of the information 
recording medium, as indicated in FIGS. 5A, 5B and 5C, by varying the 
thickness of the recording film 3. The thickness of the recording film is 
in a order that FIG. 5B&gt;FIG. 5C&gt;FIG. 5A. In the case where the information 
recording medium is irradiated with a rectangular laser light pulse for 
recording indicated in FIG. 6(a) and reproduction is effected with reading 
light whose wavelength is 830 nm, when the relation between the wavelength 
of the reading light and the reflectivity is as indicated in FIG. 5A, a 
reproduced signal indicated in FIG. 6(b) can be obtained. When the 
relation between the wavelength of the reading light and the reflectivity 
is as indicated in FIG. 5(b), a reproduced signal indicated in FIG. 6(c) 
can be obtained and further when the relation between the wavelength of 
the reading light and the reflectivity is as indicated in FIG. 5(c) a 
reproduced signal indicated in FIG. 6(d) can be obtained. 
Under the conditions that the recording film 3 of the information recording 
medium indicated in FIG. 3 is 350 nm thick, that this information 
recording medium is rotated with a speed of 1200 rpm, that after the 
recording film 3 has been initialized by irradiating the recording film 3 
with a continuous semiconductor laser light beam of a power of 14.1 mW so 
that it is melted and the elements therein react on each other, it is 
irradiated with rectangular pulses of laser diode for recording, whose 
recording frequency is 0.12 MHz, and that reading light having a 
wavelength of 830 nm is used and its reflected light is detected, a 
reproduced signal as indicated in FIG. 6(c) is obtained. Then, when it is 
irradiated with a continuous light beam of a power of 14.1 mW, the 
reproduced signal of 0.12 MHz is reduced. Therefore it is possible to 
repeat recording and erasing. The reproduced signal can be treated as it 
is without passing through any differential circuit. In addition jitter of 
the recorded signal with respect to the recording light pulse is very 
small and it is under 30 nm. Further the error rate of this signal is 
1.times.10.sup.-6. Still further, since variations in the mean 
reflectivity are small, it is recognized that a merit can be obtained that 
influences of the recording on the servo system for tracking or autofocus 
are small. Furthermore repetition of recording and erasing more than 
1.times.10.sup.5 times is possible with this information recording medium. 
After a repetition of recording and erasing of 1.times.10.sup.6 times the 
error rate is increased to 2.times.10.sup.-6, which gives rise to no 
problem in practice. 
In the case where the recording film 3 is 350 nm thick, the recording is 
possible for the recording laser power comprised between 9 and 22 mW. 
Further, for a region of the number of rotation of the information 
recording medium between 600 and 1500 rpm it is possible to obtain a 
reproduced signal having a shape similar to that obtained in this 
embodiment and the recording frequency permitting to obtain the reproduced 
signal of this embodiment is below 1 MHz at a number of rotation of 1200 
rpm. In addition, in the case where the recording frequency of the 
rectangular light pulse for recording is 0.9 MHz, a good reproduced signal 
can be obtained, when the duty cycle of the rectangular light pulse is 30 
to 70%. That is, if the duty cycle is under 30%, it is difficult to 
separate the rising and falling edges of the rectangular light pulse, and 
if it is over 70%, the cooling speed decreases and the amplitude of the 
reproduced signal is reduced. 
Under the conditions that the recording film 3 of the information recording 
medium indicated in FIG. 3 is 250 nm thick, that this information 
recording medium is rotated with a speed of 2400 rpm, the other parameters 
being kept to be same as those used in the preceding Embodiment, that 
after the recording film 3 has been initialized by irradiating the 
recording film 3 with a continuous light beam of laser diode of a power of 
14.1 mW, it is irradiated with rectangular light pulses of laser diode for 
recording, whose power is 14.1 mW and whose recording frequency is 1.77 
MHz and that reading light having a wavelength of 830 nm is used and its 
reflected light is detected, a reproduced signal as indicated in FIG. 6(d) 
is obtained. The reproduced signal coming from the part corresponding to 
the rising edge of the light pulse appears in the negative direction and 
the reproduced signal coming from the part corresponding to the falling 
edge of the light appears in the positive direction. Then, when the 
information recording medium is irradiated with a continuous light beam of 
a power of 14.1 mW, the intensity of the reproduced signal is reduced. 
Under the conditions that the recording film 3 of the information recording 
medium indicated in FIG. 3 is about 250 nm thick, that this information 
recording medium is rotated with a speed of 2400 rpm, the other parameters 
being kept to be same as those used in the preceding Embodiment, that 
after the recording film 3 has been initialized by irradiating the 
recording film 3 with a continuous light beam of a power of 14.1 mW, whose 
wavelength is 830 nm, it is irradiated with triangular light pulses for 
recording 6.7, as indicated in FIG. 7(a), whose power is 14.1 mW and whose 
recording frequency is 1.77 MHz, and that reading light having a 
wavelength of 830 nm is used and its reflected light is detected, 
reproduced signals can be obtained only at the rising edge of the 
triangular light pulse 6 and at the falling edge of the triangular light 
pulse 7. The reproduced signal coming from the recorded portion formed by 
the triangular light pulse 6 appears in the negative direction and the 
reproduced signal coming from the recorded portion formed by the 
triangular light pulse 7 appears in the positive direction. In this way 
three-valued recording is possible. Then, when the information recording 
medium is irradiated with a continuous light beam of a power of 14.1 mW, 
the reproduced signal disappears. 
In the case where the recording film 3 is 250 nm thick, recording is 
possible for a region of the power of the light pulse for recording 
between 12 and 25 mW and reproduced signals indicated in FIG. 6(d) and 
FIG. 7(b) are obtained. Further, for a region of the number of rotation of 
the information recording medium between 1800 and 3000 rpm, reproduced 
signals indicated in FIG. 6(d) and FIG. 7(b) are obtained. Still further, 
for recording frequencies under 2 MHz reproduced signals indicated in FIG. 
6(d) and FIG. 7(b) are obtained. 
Although, in the above embodiments, explanation has been made for the case 
where laser light is used as the energy beam, another light beam or 
another energy beam, e.g. an electron beam, etc. may be used. Further, 
although a recording film 3 consisting of Ge-Te-Se, for which rewrite of 
information is effected by crystal-amorphous phase change, has been used 
in the above embodiments as the recording film 3, other recording films of 
the Ge-Te system (recording films including Ge and Te, and one or a 
plurality of other elements at need) may be used and it is desirable to 
use a substance, whose crystallization time is comprised between 5 and 50 
ns. Still further it is desirable that the recording film 3 is 50-500 nm 
thick, more preferably 200-400 nm thick. In addition, although, in the 
above embodiments, the reflectivity of the information recording medium 
has been detected, it is also possible to detect light transmittance or 
polarization characteristics of the information recording medium. Further, 
although, in the above embodiments, explanation has been made for the case 
where information is recorded on one side of the information recording 
medium, it is also possible to record information on both sides of the 
information recording medium by sticking 2 sheets of the information 
recording medium indicated in FIG. 3 together using organic adhesive. 
As a modification of the case where the recording film 3 is 350 nm thick in 
this embodiment, a light pulse as indicated in FIG. 8(a) is used as the 
laser light for recording. When the recording film 3 is irradiated with 
the laser light having a recording frequency of 1 MHz and a duty cycle of 
50%, a reproduced signal as indicated in FIG. 8(b) is obtained. When it is 
irradiated with a continuous light beam of 14.1 mW as the laser light for 
erasing, the intensity of the reproduced signal is reduced, which permits 
repetitions of recording/erasing. 
Embodiment 2 
An information recording medium indicated in FIG. 3 is used, in which the 
protective layer 2 is 100 nm thick; the protective layer 4 is 200 nm thick 
and made of ZrO.sub.2 formed by the sputtering method; and the recording 
film 3 is an In-Se-Tl film formed by the co-evaporation method, these 
being disposed on a glass disk, whose diameter is 13 cm. This disk is 
sticked with another glass disk on the side of the protective layer 4 by 
means of adhesion. 
When the information recording medium, which is rotated with a speed of 
1200 rpm, is irradiated with a laser-diode light of 14.1 mW, whose 
wavelength is 830 nm, the irradiated portion of the recording film 3 is 
melted so that the different elements in the recording film 3 can react 
sufficiently on each other. The reproduction light for obtaining 
reproduced signals is a continuous light of 1.5 mW. No variations are 
observed, when the recording film is irradiated with the reading light for 
more than 100 hours. The laser light for recording is produced by a same 
laser diode as for the reading light, which laser light for recording 
consists of rectangular light pulses rising from the reproduction power 
level as indicated in FIG. 9(a). Reading out the address of the track or 
the sector is verified with irradiation at the reproduction power level 
before the rising of the pulse. When the recording film 3 is irradiated 
with the laser light for recording, whose recording frequency is 1.5 MHz 
and whose duty cycle is 50%, a reproduced signal as indicated in FIG. 9(b) 
is obtained in the same way as in Embodiment 1. Then, when it is 
irradiated with a continuous laser light of 14.1 mW for erasing, the 
intensity of the reproduced signal is reduced, which permits repetitions 
of recording/erasing. Although a recording film 3 consisting of In-Se-Tl 
has been used in the above embodiment, other recording films of the In-Se 
system (recording films including In and Se, and one or a plurality of 
other elements at need) may be used. 
Further, in a modification of this embodiment a light pulse as indicated in 
FIG. 10(a) is used as the laser light for recording. When the recording 
film 3 is irradiated with a recording light, whose recording frequency is 
1 MHz and whose duty cycle is 50%, a reproduced signal as indicated in 
FIG. 10(b) is obtained. When this is irradiated with a continuous laser 
light of 14.1 mW for erasing, the intensity of the reproduced signal is 
reduced, which permits to repeat recording/erasing. 
Finally, as the recording film 3 other than the recording film made of the 
group of materials mainly containing Ge-Te system or In-Se system, 
recording films of e.g. Ga-Se system, Sb-Se system, Sb-Te system, In-Te 
system, In-Sb system, Au-Te system, and Ga-Sb system may be used as well.