Optical recording medium and a method for reproducing information recorded from same

The optical recording medium includes a first layer which is in a state that includes different phases having different optical constants and in which information has been recorded. The second layer is capable of changing states, which states include different phases and different optical constants. There is also provided a method of reproducing information recorded on an optical recording medium which includes a first layer having different phases and different optical constants and a second layer capable of changing states, which states include different phases and different optical constants. The method employs irradiating the second layer with a laser beam, and reproducing information recorded on the first layer from an area corresponding to an area of the second layer where the optical constants change due to laser irradiation.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical recording medium such as an 
optical disc on which high density information has been recorded by means 
of a laser beam. The present invention also relates to a method for 
reproducing information recorded on such optical recording medium. 
In recent years, proposals surfaced for a Super Image Dissection 
Reproducing Technique where high density information may be reproduced 
with the use of pits smaller than a laser beam spot For instance, there 
has been a suggestion that a chalcogen amorphous material such as Sb.sub.2 
Se.sub.3 be employed, so that high density information may be reproduced 
by making use of a phenomenon called Reflectance Change which is caused by 
a reversible phase change between an amorphous state and a crystallized 
state (Japanese Patent Application Laid-open 3-292632). 
According to this prior art, a film of a phase change material is formed on 
a transparent substrate in which signal pits have already been formed in 
accordance with information signals. When the laser beam is applied to the 
phase change material, the rising temperature of the phase change material 
within the applied laser beam spot causes the reflectance to rise so as to 
form a high reflectance area (amorphous state). In reproduction, only pits 
within the high reflectance area are detected, and high density 
reproduction may be effected by making use of Reflectance Change, i.e., a 
fact that an information reproducing area is a high reflectance area and 
other areas are low reflectance areas. 
Namely, the technique disclosed in the above prior art is useful for an 
optical disc of the ROM (Read Only Memory) type which includes a 
transparent substrate having signal pits formed in advance, and a single 
layer of phase change material formed on the transparent substrate. 
However, there has not been suggested a technique useful for other types of 
optical recording medium such as the Write Once, the Read Many type, or 
the Rewritable type: with which can record information and reproduce the 
information at any desired time. 
Moreover, even with the technique being useful only for the ROM type 
optical disc, the reflectance of areas (masked areas) other than the 
information reproducing area are required to be sufficiently lower than 
that of the information reproducing area, but with respect to the focus 
servo, it is required that the masked areas should have a high 
reflectance. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve the above-mentioned problems 
peculiar to the above-mentioned prior arts, so as to provide an improved 
optical recording medium and a method for reproducing information recorded 
in such an improved optical recording medium, thereby making it possible 
to reproduce high density information recorded in a rewritable type 
optical recording medium. 
According to a first aspect of the present invention, there is provided an 
optical recording medium comprising a first layer which is in a state that 
includes different phases having different optical constants and in which 
information has been recorded, and a second layer capable of changing 
states, including different phases having different optical constants. 
According to a second aspect of the present invention, there is provided a 
method of reproducing information recorded on an optical recording medium 
which is comprised of a first layer including different phases having 
different optical constants and a second layer capable of changing to 
different states, and phases with different optical constants. The method 
comprises irradiating the second layer with a laser beam, and reproducing 
information recorded in the first layer from an area corresponding to an 
area of the second layer where the optical constants changes due to the 
laser irradiation. 
The above objects and features of the present invention will become better 
understood from the following description with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 5 is a cross sectional view indicating the structure of an optical 
recording medium of the present invention and the principle of how to 
reproduce information recorded therein. 
Referring to FIG. 5, the optical recording medium comprises a recording 
layer 14 and a reproduction auxiliary layer 13. The recording layer 14 
includes information signal areas 17 (e.g. crystallized state) and 
non-information signal areas 18 (e.g. amorphous state). The reproduction 
auxiliary layer 13 employs a material which under room temperature is in a 
stabilized crystallized state 16, but will change into an amorphous state 
15 upon being irradiated with a laser beam during reproduction. The 
amorphous state 15 is an area which has been melted to a temperature 
higher than its melting point, but will change back into its original 
stabilized crystallized state upon being cooled to a temperature below its 
melting point. 
The principle for reproducing information recorded in the above medium will 
be described as follows. 
Referring to FIG. 5, a laser beam from above the auxiliary layer 13 is 
multiple-reflected on an interface between the recording layer 14 and the 
auxiliary layer 13, produces a reflected light returning back to the space 
above the auxiliary layer 13, serving as an information reproducing signal 
or focus servo signal. With the optical recording medium shown in FIG. 5, 
there are four kinds of reflected lights. 
(I) When the reflected light is coming from the area 15 (optical constant 
N1) of the reproduction auxiliary layer 13 and the area 17 (optical 
constant N3) of the recording layer 14, the reflected light has a 
reflectance of A %. 
(II) When the reflected light is coming from the area 15 (optical constant 
N1) of the reproduction auxiliary layer 13 and the area 18 (optical 
constant N4) of the recording layer 14, the reflected light has a 
reflectance of B % (B is not equal to A) 
(III) When the reflected light is coming from the area 16 (optical constant 
N2) of the reproduction auxiliary layer 13 and the area 17 (optical 
constant N3) of the recording layer 14, the reflected light has a 
reflectance of C %. 
(IV) When the reflected light is coming from the area 16 (optical constant 
N2) of the reproduction auxiliary layer 13 and the area 18 (optical 
constant N4) of the recording layer 14, the reflected light has a 
reflectance of C %. 
It is understood that within an area irradiated by a laser beam spot as 
shown in FIG. 5, reflecting light percentages as described in categories 
(I) and (II) are different due to different reflectances of each area, and 
such a difference in light reflecting percentages can serve as a signal 
for detection, thus making it possible to reproduce information recorded 
in the recording layer 14. This may also be explained as follows. Due to 
multiple optical interference between the recording layer 14 and the 
reproduction auxiliary layer 13, reflected lights from certain areas will 
become, comparatively speaking, stronger, and reflected lights from other 
areas will become, comparatively speaking, weaker. 
As indicated in FIG. 5, the quantity of reflected light in above category 
(T) is different from that in category (II), thereby making it possible to 
detect information signals in the area of optical constant NI on the 
reproduction auxiliary layer 13. On the other hand, while the laser beam 
is being applied to the area 16 of optical constant N2 on the reproduction 
auxiliary layer 13, no information signal is able to be detected because 
the quantities of all the reflected lights will be almost the same no 
matter whether they are reflected from the area 17 of optical constant N3, 
or the area 18, of optical constant N4. 
FIGS. 1a-1c indicate an optical recording medium in which a reproduction 
auxiliary layer 3 is in a crystallized state at room temperature. An area 
being irradiated by a laser beam will generally be of a circular form, but 
the high rotating speed of the optical recording medium will cause the 
circular form to become elliptical in form. This is an area having a 
temperature higher than its melting point. 
FIGS. 2a and 2b are graphs each showing a relationship between the 
thickness of a recording layer and the reflectance of a light reflected 
therefrom. As understood in FIGS. 1a, 1b and FIG. 2a, when the thickness 
of a recording layer 4 is 200 angstrom, in a reproducing area 8 which is 
covered by both the laser beam spot and the high temperature area 7 and 
corresponds to the amorphous area of the reproduction auxiliary layer 3, 
there will be a reflectance variation due to different areas (crystallized 
state and amorphous state), thus ensuring high density information 
reproduction. 
FIGS. 3a-3c indicate another optical recording medium. The reproduction 
auxiliary layer 11 in FIG. 3b is in an amorphous state under room 
temperature. Also, an area of elliptic form having a temperature higher 
than the melting point is formed extending from the center of the laser 
beam spot, and a crescent-shaped reproducing area 9, having a 
crystallizing temperature, is formed before the elliptic high temperature 
area. 
As shown in FIGS. 3a, 3b, and 4b, when the thickness of the recording layer 
12 is 130 angstrom, in an area of the recording layer 12 corresponding to 
a crystallized area of the reproduction auxiliary layer 11, there will be 
a reflectance variation due to different areas (crystallized area 6 and 
amorphous area 5 of the recording layer 12), thus ensuring that higher 
density recorded information, than that shown in FIG. 2a, may be 
reproduced from a reproducing area 9 (crystallized area) covered by a 
laser beam spot. 
The present invention will be described in more detail by way of the 
following examples. 
EXAMPLE 1 
Referring to FIG. 1b, an optical recording medium in this example comprises 
a transparent substrate 2, a reproduction auxiliary layer 3 formed 
immediately under the substrate 2, and a recording layer 4 formed under 
the auxiliary layer 3. 
The reproduction auxiliary layer 3 is made of germanium/tellurium Ge--Te 
(when in its crystallized state n=5.8, k=3.6; when in its amorphous state 
n=4.3, k=O. 95. n: index of refraction, k: extinction coefficient). The 
material constituting the layer 3 is in a stabilized crystallized state at 
room temperature, but will change into an amorphous state when melted. 
Furthermore, as soon as the temperature is below melting point, it will 
quickly change back to the stabilized crystallized state. Layer 3, in this 
example, is composed of a material reversibly changeable between a 
crystallized state and an amorphous state, but it is also possible to use 
another material such as indium/antimony In--Sb, which is reversibly 
changeable from one crystallized state into another crystallized state. 
The recording layer 4 is made of germanium/antimony/tellurium Ge.sub.2 
Sb.sub.2 Te.sub.5 (when in its crystallized state n=5.2, k=3.4; when in 
its amorphous state n=4.9, k=1.35). While at reproduction and room 
temperatures, the material will be in two states, i.e., a crystallized 
state and amorphous state. An area having information recorded therein may 
be in either a crystallized area 5 or an amorphous area 6 (FIG. 1b). 
Similar to reproduction auxiliary layer 3, which is used as the layer 4 in 
this example is a material reversibly changeable between a crystallized 
state and an amorphous state, but it is also possible to use another 
material which is reversibly changeable from one crystallized state to 
another crystallized state. 
A difference between the reproduction auxiliary layer 3 and the recording 
layer 4 is that, an area of the layer 3 being irradiated by a laser beam 
is in an amorphous molten state but will change back to its original 
crystallized state once the laser beam moves away, whilst in the recording 
layer 4, after the laser beam has moved away both the amorphous state and 
crystallized state will remain without any change. 
Although, it is apparent that the optical recording medium comprises a 
transparent substrate 2, a reproduction auxiliary layer 3 and a recording 
layer 4, dielectric layers (made of a material such as ZnS.SiO.sub.2), may 
be interposed between the above layers for the purpose of cooling or heat 
accumulation. Since a certain amount of heat will be generated due to the 
laser beam irradiation the dielectric layers thus interposed are 
especially important in prohibiting transfer of heat to the recording 
layer 4. 
The reproduction of the information recorded on the above-described optical 
recording medium may be explained as follows. 
Referring to FIG. 1a, a laser beam spot having a circular form is produced 
by a light source having a light wave length of 780 nm and a numerical 
aperture of 0.45, the light reflected from a reproducing area 1 (for 
instance in an amorphous state even under room temperature) corresponding 
to a pit of a CD (compact disc) may be read so as to reproduce information 
recorded therein. Referring to FIG. 1b, when reading high density 
information recorded by numerous signal pits having a size of about 
0.4.mu., a laser beam having a spot size of 1.7.mu. may be adjusted in its 
intensity and wave length, so that the high temperature portion (molten 
part having a temperature higher than the melting point, see FIG. 1c) of 
the beam spot may be adjusted to 0.8 .mu.m, thereby obtaining a more 
reliable and effective laser beam spot. 
Referring again to FIG. 1a, an amorphous molten area 7 exhibits a circular 
form when the optical recording medium is in a stopped condition, but the 
high rotating speed of the optical recording medium will elongate the 
circular form of the laser beam into an elliptical form extending from the 
center of the beam spot. Referring to FIGS. 1a and 1b, the common portions 
of the elliptical molten area 7 and the circular laser beam spot area will 
serve as an information signal reproducing area 8 (corresponding to the 
blank portion of the reproduction auxiliary layer 3), whilst other areas 
(corresponding to the hatching portions of the reproduction auxiliary 
layer 3) are masked. 
Here, an area of the reproduction auxiliary layer 3 corresponding to the 
information signal reproducing area 8 is in an amorphous state. 
Furthermore, the thickness of the reproduction auxiliary layer 3 is limited 
by the extinction coefficient k, the thickness of the layer 13 in this 
example is 50-500 angstrom, preferably 100 angstrom. 
When the thickness of the reproduction auxiliary layer 3 is set to be 100 
angstrom and the thickness of the recording layer 4 is set to be 200 
angstrom, there will be a reflectance difference of about 10% between the 
crystallized area and the amorphous area of the recording layer 4 
corresponding to information signal reproducing area 8. Accordingly, by 
detecting lights reflected from such an optical recording medium, it is 
possible to reproduce high density information, having a pit size of about 
0.4.mu., recorded on the recording layer 4. 
However, in areas other than he information signal reproducing area 8, 
since there are almost no reflectance differences between the crystallized 
and amorphous portions of the recording layer 4, it is impossible to 
reproduce any information therefrom. 
Referring to FIG. 2a, when the thickness of the recording layer 4 is 200 
angstrom, the reflectance of a crystallized area is greater than that of 
an amorphous area. However, it is also possible to use a material whose 
crystallized area has a smaller reflectance than its amorphous area. In 
fact, the thickness of the recording layer 4 can be in a range of 180-400 
angstrom. In any condition with the recording layer 4, a reflectance ratio 
(a difference between a maximum reflected light and a minimum reflected 
light with respect to a shortest pit length: a difference between a 
maximum reflected light and a minimum reflected light) should be at least 
30%. 
EXAMPLE 2 
Referring to FIG. 3b, an optical recording medium in this example comprises 
a transparent substrate 10, a reproduction auxiliary layer 11 formed 
immediately under the substrate 10, and a recording layer 12 formed under 
the auxiliary layer 11. 
The reproduction auxiliary layer 11 is made of germanium/antimony/tellurium 
Ge.sub.2 Sb.sub.2 Te.sub.5 (when in its crystallized state n=5.2, k3.4 
when in its amorphous state n=4.9. k=1.3). The material constituting the 
layer 11 is a phase change material which is in a stabilized amorphous 
state under room temperature or at a temperature higher than its melting 
point. For instance, at room temperature the reproduction auxiliary layer 
11 as a whole is in a solid amorphous state, but will change into a molten 
amorphous state upon being irradiated by a laser beam. Additionally, as 
indicated in FIG. 3c, a portion having a crystallizing temperature lower 
than its melting point will occur adjacent to the molten area. As soon as 
the laser beam spot moves away, the irradiated portion (molten amorphous 
area) will change back to a solid amorphous state, and the crystallized 
areas will also change to a solid amorphous sate. 
The recording layer 12 is made of indium/silver/antimony/tellurium InAgSbTe 
(when in its crystallized state n=4.04, k=3.28; when in its amorphous 
state n=4.5, k=1.48). At both reproduction and room temperatures, the 
material will be in two states, i.e., a crystallized state and an 
amorphous state. An area 9 having information recorded therein may be 
either a crystallized area or an amorphous area (FIG. 3b). The material 
used as the layer 12 in this example, is reversibly changeable between a 
crystallized state and an amorphous state, but it is also possible to use 
another material reversibly changeable from one crystallized state to 
another crystallized state. The recording layer 12, in this example, after 
the laser beam spot has moved away, both the amorphous state (for 
instance, having information recorded therein) and crystallized state will 
remain in their present form without any change. 
Although, the optical recording medium, in this example, comprises a 
transparent substrate 2, a reproduction auxiliary layer 3 and a recording 
layer 4, it is allowable to interpose a dielectric material between the 
above layers for the purpose of cooling or heat accumulation. 
The reproduction of the information recorded in the above-described optical 
recording medium may be explained as follows. 
By adjusting the power intensity of a recording laser beam, a molten 
portion will be formed within the laser beam spot. Referring again to FIG. 
3a a molten (amorphous) area 7 has a circular form when an optical 
recording medium is in a stopped condition, but the high rotating speed of 
the optical recording medium will cause the circular form to be elongated 
into an elliptical form extending from the center of the beam spot. 
Here, an area 9 having a crystallizing temperature lower than its melting 
point will occur adjacent to the molten (amorphous) area within the beam 
spot, as indicated in FIGS. 3a-3c. Therefore, the information signal 
recorded in the recording layer 12 may be reproduced at a high density, 
from the area 9 having such a crystallizing temperature. 
Furthermore, as shown in FIG. 3b, the edge portions of the crystallizing 
temperature area 9 and the inner portion of the laser beam spot are in an 
amorphous (molten) state, and the information signal of the recording 
layer 12 will be masked by way of multiple optical interference, just as 
are the areas (solid amorphous state) outside of laser beam spot. 
Therefore, it is possible to prevent the deterioration of a MTF 
(modulation transfer function) which can be caused due to a variation in 
the power of the laser output. Moreover, as illustrated in FIG. 3a, since 
the crystallizing area 9 has a crescent form, it is possible to reproduce 
an information signal at a higher density than the optical recording 
medium described in example 1. 
In addition, referring to FIGS. 3a-3c, the molten (amorphous) area 7 will 
become larger or smaller corresponding to the variation in the laser power 
output, and such a variation will also cause a shift of the crystallizing 
temperature area 9 in the beam scanning direction. However, it has been 
proven that the area 9 will maintain a stabilized width without any 
change, thereby ensuring stabilized and reliable information reproduction. 
In this example, the information reproducing area 9 of the reproduction 
auxiliary layer 11 is in its crystallized state, the thickness of the 
layer 11 is 100 angstrom. However, the recording layer 12 comprises a 
crystallized area and an amorphous area. 
In this example, when (i) the reproducing area 9 is in its crystallized 
state, (ii) other areas within the laser beam spot are in their molten 
amorphous state, (iii) the areas outside of the laser beam spot are in a 
solid amorphous state, (vi) the thickness of the recording layer 12 is 130 
angstrom, the reflectance difference between crystallized area and the 
amorphous area of the recording layer 12 will be about 10% as indicated in 
FIG. 4b. Therefore, from the detecting light reflected from such an 
optical recording medium it is possible to reproduce, in high density, the 
information signal of 0.4.mu. or less recorded in the recording layer 12. 
In an area within the laser beam spot but outside of the information 
reproducing area 9, the light reflectance will be 25%. But, since there is 
almost no reflectance difference between crystallized and amorphous areas 
of the recording layer 12 corresponding to such an area, it is impossible 
to effectuate any information reproduction. 
In this example, the thickness of the recording layer 12 is 130 angstrom, 
the reflectance of an amorphous area is greater than that of a 
crystallized area. However, it is also possible to use a material whose 
amorphous area has a smaller reflectance than its crystallized area. In 
fact, the thickness of the recording layer can be in a range of 50-150 
angstrom. In any condition with the recording layer 12 a reflectance 
ratio, as in example 1, should be at least 30%. 
In this example, it is preferable that the thickness of the recording layer 
12 be 130 angstrom. This is because when the thickness of the recording 
layer 12 is 130 angstrom, corresponding to the crystallized area of the 
reproduction auxiliary layer 11 which is within the laser beam spot, 
information reproduction is possible due to the reflectance difference as 
shown in FIG. 4b. On the other hand, corresponding to the amorphous molten 
area of the reproduction auxiliary layer 11 which is also within the laser 
beam spot, information reproduction is impossible because no reflectance 
difference exists as indicated in FIG. 4a. 
In both, example 1 and example 2, the materials which have been used to 
form a recording layer and a reproduction auxiliary layer are Ge--Te, 
In--Sb, Ge.sub.2 Sb.sub.2 Te.sub.5, InAgSbTe. However, it is possible to 
use, as a phase change material reversibly changeable between an amorphous 
state and a crystallized state, TeTeO.sub.2 GeSn, TeGeSnAu, GeTeSn, 
GeTeSbS, SnSeTe, SbSeTe, SbSe, GaSeTe, GaSeTeGe, InSe, InSeTICo, GeTeSb. 
As a material reversibly changeable from one crystallized state to another 
crystallized state, it is possible to use AgZn, CuAINi, InSbSe, InSbTe. 
Furthermore, in both the examples 1 and 2, it is also possible that the 
recording layers and reproduction auxiliary layers contain some organic or 
inorganic coloring materials, and some photochromic materials capable of 
changing from one (optical constant) state to another without any thermal 
reactions. 
As understood in the above description, since the optical recording medium 
and the reproduction method thereof according to the present invention, 
are suitable for reproducing at high density the information recorded in 
optical recording medium, they are also suitable for use with an optical 
recording medium of the Write Once Read Many type and the Rewritable type, 
Also, by providing an information reproducing area and a masked area 
respectively, it is possible to increase the reflectance of the masked 
area so as to ensure the necessary amount of reflected light for the focus 
servo, and at the same time to ensure the necessary reflectance for 
information reproduction. 
The reflectance of the masked area may be easily controlled by controlling 
the thicknesses of the recording layer and the reproduction auxiliary 
layer. 
While the presently preferred embodiments of the this invention have been 
shown and described above, it is to be understood that these disclosures 
are for the purpose of illustration and that various changes and 
modifications may be made without departing form the scope of the 
invention as set forth in the appended claims.