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CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to application Ser. No. 08/356,589, filed Dec.
15, 1994.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium which can
record and reproduce information in high density and a reproducing method
therefor.
2. Description of the Background Art
In relation to an optical disk unit and an optical medium, the so-called
MSR (magnetically induced super resolution) system has recently been
watched with interest as a super resolution means for
recording/reproducing information in/from regions smaller than a recording
spot which is defined by the diffraction limit of light. Such an MSR
system is disclosed in Technical Digest of Optical Data Storage Topical
Meeting, 1991, Vol. 5, pp. 112 to 115 (Lecture No. TUB-3) and pp. 116 to
119 (Lecture No. TUB-4), for example. This system is characterized in that
a plurality of magneto-optical recording films are employed with provision
of a masking layer for masking peripheral information other than that for
an information recording layer. Recorded information (recording marks) is
transferred to the masking layer, whose temperature is increased by
irradiation, by means of a relatively intense reproducing beam, whereby
influences, caused by adjacent tracks and recording marks, in the linear
density direction, are suppressed even if the recording density is
increased, and therefore the optical resolution is improved.
On the other hand, Japanese Patent Laying-Open No. 5-225611 (1993), for
example, discloses an optical recording medium which is provided with a
masking layer including a light absorption center causing a nonlinear
light absorption phenomenon such as a saturable absorption property, as an
optical recording medium for attaining a similar super-resolution effect.
Further, Japanese Patent Laying-Open No. 5-242524 (1993) discloses a
recording/reproducing method similarly utilizing a nonlinear optical
phenomenon, employing a spiro-selenazolino-benzopyran which exhibits
inverse photochromism. In addition, Japanese Patent Laying-Open No.
5-266478 (1993) proposes a method employing a masking layer which is
generally non-transmittable with respect to a reproducing beam, but
exhibits partial transmission only in its central portion upon irradiation
with a reproducing beam, which is controlled to exceed prescribed
intensity on the central portion and again returns to the opaque state
after passage of the reproducing beam. This reference discloses using an
indoline spiropyran which exhibits inverse photochromism as a material for
the masking layer.
In the conventional method employing the MSR system, however, the recording
medium is disadvantageously limited to a magneto-optical recording medium.
Further, the method employing the nonlinear light absorption phenomenon
generally requires extremely high light intensity, and hence information
which is already recorded in the recording layer may be destroyed by heat
generated when new or further information is applied.
In the super-resolution system utilizing inverse photochromism, in
addition, reproduction is carried out by scanning the recording layer with
a laser beam spot for decoloring the masking layer by a photochromic
reaction. However, the masking layer thereafter naturally returns to a
colored state due to a thermal reaction, and hence it is difficult to
attain compatibility with a medium having no masking layer, i.e.,
employing no super-resolution optical recording technique. When the beam
is impinged on the medium in reproduction, further, a coloring reaction is
also caused by a thermal reaction with progress of decoloration by a
photochromic reaction, and hence transmittance of the masking layer may be
insufficiently improved.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical recording
medium which does not require an extremely high light intensity dissimilar
to saturable absorption, wherein the light intensity is sufficiently low
that information recorded in its recording layer is not, destroyed by such
heat as may be generated, and capable of attaining compatibility with an
optical disk employing no super-resolution effect, and a drive therefor,
which enables high-density recording/reproduction of information by a
super-resolution effect.
An optical recording medium according to a first aspect of the present
invention comprises a recording layer, and a masking layer which is
converted to a state having low absorption at the wavelength of a
reproducing beam due to decoloration which is caused by a thermal reaction
upon irradiation with the reproducing beam.
An optical recording medium according to a second aspect of the present
invention comprises a recording layer, and a masking layer which is
converted to a state having low absorption at the wavelength of a
reproducing beam due to facilitation of decoloration caused by a photon
mode photochromic reaction through temperature rise upon irradiation with
the reproducing beam.
In the optical recording medium according to the present invention, the
masking layer is converted to a state having large absorption at the
wavelength of the reproducing beam by a photon mode photochromic reaction
upon irradiation with a beam of a specific wavelength which is different
from the reproducing beam.
According to the present invention, the masking layer can be provided on
the side of the recording layer directed toward the reproducing beam. The
present invention may be applied to both a reflection type system and a
transmission type, and the masking layer may be provided over or under the
recording layer.
The optical recording medium according to the second aspect of the present
invention is provided with the masking layer, which is facilitated in
decoloration by a photon mode photochromic reaction through temperature
rise upon irradiation with the reproducing beam, and is converted to a
state having low absorption at the wavelength of the reproducing beam, as
the result. This masking layer contains a photochromic material which is
activated for photon mode reactivity by the temperature rise generated
upon irradiation with the reproducing beam. Well-known spiropyran
photochromic materials cannot be employed in the second aspect of the
present invention because they exhibit coloration and/or achormatization
as a result of a thermal reaction. Examples of a photochromic material
employed in the second aspect of the present invention are a fulgide
photochromic compound described later, and a diarylethene photochromic
material expressed in the following general formula, for example:
##STR1##
where R.sub.1 to R.sub.5 each represent an alkyl group, a halogen atom, a
hydrogen atom, a trifluoromethyl group, an alkoxy group, a cyano group, an
amino group or a dimethylamino group, and B represents a hydrocarbon ring
or a heterocyclic ring.
Examples of the diarylethene material are compounds having the following
structural formulas:
##STR2##
FIG. 7 illustrates temperature dependency levels of reaction sensitivity of
the above diarylethene compounds. The abscissa shows the reciprocal of
absolute temperature, and the ordinate shows quantity of irradiation light
required for halving absorption. It is clearly understood from FIG. 7 that
the quantity of irradiation light required for halving absorption is
reduced as the temperature is increased, i.e., values on the abscissa are
reduced and quantum yields are increased by raising the temperature,
thereby increasing photochromic reaction sensitivity levels. Thus, these
compounds are facilitated in photon mode photochromic reaction by raising
the temperature. These compounds cause no decoloration even if the same
are heated by the absorption of light. Thus, it is understood that no
decoloration is caused by a thermal reaction. Thus, the photochromic
material which is employed for the masking layer according to the second
aspect of the present invention is preferably prepared from a material
which does not cause either substantial decoloration or coloration by a
thermal reaction, so that there is substantially no change in the
absorption of the masking layer at the wavelength of the reproducing beam.
A reproducing method according to a third aspect of the present invention
is adapted to irradiate the recording layer of the optical recording
medium according to the first aspect of the present invention with a
reproducing beam for reproducing information recorded therein, and
comprises the steps of irradiating the masking layer with a first beam of
a specific wavelength for reducing transmittance of the masking layer at
the wavelength of the reproducing beam, and causing a thermal reaction by
irradiating the masking layer having reduced transmittance, with the
reproducing beam (a second beam), whose wavelength is different from the
specific wavelength of the first beam whereby increase transmittance of a
part of the masking layer corresponding to a reproducing beam spot, and
irradiating the recording layer with the reproducing beam.
A reproducing method according to a fourth aspect of the present invention
is adapted to irradiate the recording layer of the optical recording
medium according to the second aspect of the present invention with a
reproducing beam for reproducing information recorded therein, and
comprises the steps of irradiating the masking layer with a beam, of a
first specific wavelength for reducing transmittance of the masking layer
at the wavelength of the reproducing beam, and causing a photon mode
photochromic reaction which is facilitated by temperature rise by
irradiating the masking layer having reduced transmittance with the
reproducing beam whose wavelength is different from the specific beam
wavelength of the first (a second beam), whereby to increase transmittance
of a part of the masking layer corresponding to a spot of the reproducing,
second beam, beam and irradiating the recording layer with the
reproducing, second beam.
In the reproducing method according to the present invention, the first
beam of a specific wavelength has a spot size which is larger than that of
the reproducing, second, beam in general.
In order to attain a super-resolution effect by providing the masking
layer, it is necessary to cause an increase of the transmittance of the
masking layer, i.e., nonlinear reduction of absorptivity by irradiation
with the beam. The super-resolution effect is increased as such
nonlinearity is increased.
It was found that in the super-resolution reproducing system employing an
photochromic material, of which decoloration sensitivity through a photon
mode photochromic reaction does not change with a change in its
temperature, for the masking layer, this nonlinearity is caused by using a
masking layer having a high optical density, i.e., at low transmittance.
FIG. 1 shows changes of masking layer transmittance values with respect to
quantity of irradiated light. A curve A shows a transmittance change in a
masking layer which is formed of such a photochromic material.
According to the first aspect of the present invention, the masking layer
is made of a material which is converted to a state having low absorption
at the wavelength of the reproducing beam by means of a thermal reaction.
In such a reaction, the thermal decoloration is abruptly caused when the
temperature exceeds a constant level, which is a threshold value in
general. Curve B in FIG. 1 shows a transmittance change in a masking layer
according to the present invention. It is understood that higher
nonlinearity of transmittance change is attained in this case.
According to the second aspect of the present invention, the masking layer
is made of a material which is converted to a state having small
absorption at the wavelength of the reproducing beam by a photon mode
photochromic reaction facilitated by temperature rise. Also in the case of
this reaction, photon mode decoloration abruptly progresses at a
temperature exceeding a constant level while irradiating with a
reproducing beam, to attain large nonlinearity as shown in the curve B in
FIG. 1.
In the first aspect of the present invention, the decoloration is caused by
a thermal reaction and hence decoloration of the masking layer may
simultaneously take place by a thermal reaction which is caused upon
irradiation with a coloring beam having a specific wavelength,. to result
in an insufficient masking effect. In order to prevent this, it is
necessary to increase the coloring spot for reducing energy density, while
the quantity of irradiation light may be so insufficient that coloring is
insufficient when the energy density is reduced. In the second aspect of
the present invention, the decoloration caused by a photon mode
photochromic reaction which is facilitated by temperature rise as well as
the coloration caused by a photon mode photochromic reaction are employed
and hence it is possible to make the masking layer cause neither
coloration nor decoloration by only a thermal reaction. Thus, no
decoloration is caused by heat generated upon irradiation with a coloring
beam and no coloration is caused by heat generated upon irradiation with a
reproducing beam, whereby a high super-resolution effect can be attained
with no problem in the first aspect described above.
On the other hand, it is also possible to bring the masking layer of the
optical recording medium according to the second aspect of the present
invention into an optically transparent state by irradiating it with a
decoloration beam, since coloration and decoloration progress by a photon
mode photochromic reaction. Such an optically transparent state can be
maintained by storing the medium in a cassette which does not transmit
light of the required wavelength. Therefore, it is possible to handle the
optical recording medium similarly to that provided with no masking layer,
thereby maintaining compatibility.
In the optical recording medium according to the present invention, the
masking layer can be colored due to large absorption at the wavelength of
the reproducing beam caused by a photon mode photochromic reaction upon
irradiation with a beam of a specific wavelength which is different from
the reproducing beam.
In one of preferred modes of the present invention, the spot size of a
coloring beam is made larger than the spot size of a reproducing beam.
FIG. 2 is a plan view showing the relation between a reproducing beam spot
and a coloring beam spot in the said one of the preferred modes of the
present invention. Referring to FIG. 2, a coloring beam spot 1 is applied
in advance of a reproducing beam spot 3, to form a colored region 2 in a
masking layer by the coloring beam spot 1. The colored region 2 is formed
since absorption at the wavelength of the reproducing beam is increased by
the coloring beam spot 1 having a specific wavelength due to a
photochromic reaction. This coloring beam spot 1 is sized to be larger
than the reproducing beam spot 3 and to include recording marks 14 of a
track which is adjacent to recording marks 13 of a reading track. Such a
setting leads to masking the recording marks of the adjacent track and
preventing it from causing crosstalk upon reproducing.
The reproducing beam spot 3 moves in the same direction as the coloring
beam spot 1, so that a rear half 4 of its central portion has particularly
large quantity of irradiation light and a high temperature. Therefore, in
the rear half 4, a photon mode photochromic reaction which is facilitated
by temperature rise or a thermal reaction progresses to reduce absorption
at the wavelength of the reproducing beam and to improve transmittance.
Therefore, the transmittance is so increased in the rear half 4 of the
reproducing beam spot 3 that the recording layer is irradiated with the
reproducing beam at the rear half 4 which is smaller than the reproducing
beam spot 3. Thus, influences by the recording marks 14 of the adjacent
track are masked and it is possible to reduce the track pitch. Referring
to FIG. 2, numeral 12 denotes recording marks as read. While the masking
layer remains in the state having high transmittance after reading of the
recording marks as shown in FIG. 2, the coloring beam spot 1 is applied in
advance of the reproducing beam spot 3 when the reproducing beam spot 3
moves to the adjacent track. Therefore, the masking layer is colored by
the coloring beam spot 1 and no influences are exerted by the read
recording marks when information is read from the adjacent track.
When a coloring beam spot is applied in advance of a reproducing beam spot
for coloring a masking layer, decoloration may disadvantageously be caused
simultaneously with coloration by heat which is applied by the coloring
beam spot since the masking layer according to the first aspect of the
present invention is decolored by heat. Also in consideration of this
point, it is preferable to set the coloring beam spot at a size which is
larger than the reproducing beam spot as shown in FIG. 2, for reducing the
quantity of heat as generated by dispersing energy caused by the coloring
spot. As hereinabove described, the masking layer according to the second
aspect of the present invention is facilitated in decoloration through a
photon mode photochromic reaction by temperature rise and coloration
through a photon mode photochromic reaction and causes no decoloration
directly by temperature rise, and hence no decoloration is caused in the
masking layer upon irradiation with a coloring beam spot. Thus, it is
possible to attain sufficient coloring by the coloring beam spot, thereby
attaining a sufficient super-resolution effect.
The foregoing and other objects, features, aspects and advantages of the
present invention will become more apparent from the following detailed
description of the present invention when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram for illustrating nonlinear changes of masking layer
transmittance values with respect to quantity of irradiation light;
FIG. 2 is a plan view for illustrating a reproducing method according to a
mode of the present invention;
FIG. 3 illustrates absorption spectra of a photochromic material employed
in an embodiment of the present invention;
FIG. 4 illustrates the structure of an optical recording medium according
to the embodiment of the present invention;
FIG. 5 illustrates frequency characteristics of reproducing signals in
inventive and comparative samples;
FIG. 6 illustrates absorption spectra of another photochromic material
employable in the present invention;
FIG. 7 illustrates temperature dependency values of reaction sensitivity
levels of diarylethene photochromic compounds which are employable for a
masking layer according to the present invention;
FIG. 8 is a sectional view showing the structure of an optical recording
medium according to another embodiment of the present invention;
FIG. 9 is a block diagram showing a recording/reproducing apparatus
employed in Example of the present invention; and
FIG. 10 shows relations between frequency characteristics and outputs in
reproduction of information from optical recording media according to
Example of the present invention and comparative examples.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 4 is a sectional view showing an optical recording medium according to
an embodiment of the first aspect of the present invention. Referring to
FIG. 4, a masking layer 102 is provided on a transparent substrate 101.
The masking layer 102 contains molecules of a photochromic material which
causes a photochromic reaction upon irradiation with a coloring beam and
is reduced in transmittance as the result, while causing decoloration by a
photochromic reaction facilitated by thermal reaction or heat upon
irradiation with a reproducing beam and being reduced in transmittance. A
recording layer 103 and a protective layer 104 are successively formed on
the masking layer 102. The material for the recording layer 103 can be
selected from various ones such as well-known magneto-optical materials
and phase change materials, while a TbFeCo-based magneto-optical material
is employed in this embodiment. The masking layer 102 is prepared from a
spiropyran photochromic material having the following molecular structure:
##STR3##
FIG. 3 illustrates absorption spectra of the photochromic material having
the aforementioned structure. Referring to FIG. 3, curves (1), (2) and (3)
show absorption spectra in an unirradiated state, after irradiation with a
beam of 350 nm, and after irradiation with a beam of 500 nm or after heat
treatment, respectively. As shown in FIG. 3, this photochromic material
has absorption in the ultraviolet wavelength region in an decolored state,
and is converted to a colored state having absorption in the visible
wavelength region upon irradiation with ultraviolet light (UV). While the
photochromic material can be converted from the colored state to the
decolored state by a photochromic reaction through irradiation with
visible light (VIS), its sensitivity is low and can be converted to the
decolored state mainly by a thermal reaction. In this embodiment, the
masking layer 102 is decolored through a change caused by the thermal
reaction, in accordance with the first aspect of the present invention.
It is possible to form the masking layer 102 by mixing the aforementioned
photochromic material into polystyrene resin, dissolving the mixture in
cyclohexanone and applying the same by spin coating. According to this
embodiment, the masking layer 102 has a thickness of 0.3 .mu.m. In such
formation of the masking layer 102 by spin coating, the polystyrene resin
may be replaced by another resin, while only a coloring matter may be
dissolved in a solvent to form a thin film. Alternatively, the masking
layer 102 may be formed by vacuum-depositing a coloring matter. The
thickness of this masking layer 102 is preferably set to be smaller than
the depth of focus (about 1 .mu.m in general) of a reproducing laser beam
spot.
An inventive sample of the embodiment shown in FIG. 4 was prepared by
employing the transparent substrate 101 of a glass disk having a thickness
of 1.2 mm, the recording layer 103 having a thickness of 0.1 .mu.m, and
the protective layer 104 of generally used ultraviolet setting resin
having a thickness of 10 .mu.m.
Various wavelengths were previously recorded in the recording layer 103 in
a magnetic field modulation system, and information was reproduced by a
method according to a fourth aspect of the present invention. A coloring
beam was prepared by condensing an Ar laser beam having a wavelength of
360 nm, which was emitted from an ultraviolet laser, to a spot size of 2.0
.mu.m. A reproducing beam was prepared by condensing a semiconductor laser
beam having a wavelength of 630 nm to a spot size of 1.25 .mu.m. The
coloring beam power was 2.5 mW, the reproducing beam power was 1.5 mW, and
the linear velocity was 1.4 m/sec. A comparative sample of an optical
recording medium was prepared similarly to the inventive sample, except
that the same was provided with no masking layer.
FIG. 5 shows frequency characteristics of reproducing signals in the
inventive and comparative samples, with reference to output values (0 dB)
at a low frequency. As clearly understood from FIG. 5, the outputs were
reduced by 6 dB at frequencies of 0.85 MHz (corresponding to a mark length
of 0.82 .mu.m) and 1.7 MHz (corresponding to a mark length of 0.41 .mu.m)
in the comparative and inventive samples respectively. Thus, it has been
proved that the inventive optical recording medium was improved in linear
recording density by about twice as compared with the comparative sample.
Further, track-to-track crosstalk at a track pitch of 0.6 .mu.m was -15 dB
in the comparative sample, while that in the inventive sample was reduced
to about -33 dB. As clearly understood from this, it is possible to
improve the track density in addition to the linear recording density
according to the present invention.
FIG. 8 is a sectional view showing an optical recording medium according to
the second aspect of the present invention. Referring to FIG. 8, a
dielectric layer 205, a masking layer 202, another dielectric layer 206, a
recording layer 203 and a reflective layer 204 are successively stacked on
a transparent substrate 201. The transparent substrate 201 is formed by a
polycarbonate plate having a thickness of 1.2 mm. The dielectric layer 205
is formed by an AlN film having a thickness of 0.04 .mu.m, which is
prepared by sputtering. The masking layer 202 is formed by a polystyrene
resin film having a thickness of 0.1 .mu.m, which contains a diarylethene
compound having a structure expressed in the following formula. The
dielectric layer 206 is formed by an AlN film having a thickness of 0.05
.mu.m, which is prepared by sputtering. The recording layer 203 is made of
TbFeCo, which is a magneto-optical material, to have a thickness allowing
light transmission, for example 0.05 .mu.m. The reflective layer 204 is
prepared by forming an Al film by sputtering and forming a protective
layer of ultraviolet setting resin thereon. A sample of this optical
recording medium was prepared (Example).
##STR4##
For the purpose of comparison, an optical recording medium was prepared in
a structure similar to that shown in FIG. 8, except that its masking layer
contained indoline spiropyran exhibiting inverse photochromism, which is
disclosed in Japanese Patent Laying-Open No. 5-266478 (1993) (comparative
example 1):
Further, another comparative optical recording medium was prepared in a
structure similar to that shown in FIG. 8, except that its masking layer
contained a diarylethene photochromic material which is expressed in the
following formula, as a compound causing no thermal reaction with a photon
mode reaction having substantially no temperature dependency (comparative
example 2):
##STR5##
Information was recorded in and reproduced from the optical recording media
prepared in the aforementioned manner by a recording/reproducing apparatus
shown in FIG. 9. Referring to FIG. 9, a masking layer of an optical disk
20 is in an decolored state in information recording, so that the optical
disk 20 is irradiated with a laser beam having a wavelength of 670 nm
which is emitted from a semiconductor laser 32. The laser beam which is
emitted from the semiconductor laser 32 passes through a collimator lens
31, a polarized beam splitter 30 and a dichroic mirror 29, and is
condensed on a recording layer of the optical disk 20 by an objective lens
23, for heating the recording layer. A magnetic field modulation coil 21
is provided on an opposite side of the optical disk 20 for changing an
applied magnetic field in response to a recording signal, so that magnetic
field modulation recording is carried out by actions of these elements.
In information reproduction, on the other hand, a coloring beam spot having
a wavelength of 365 nm is emitted from an HeCd laser 28. This coloring
laser beam is applied onto the optical disk 20 by the objective lens 23,
through an ND filter 27, a control element 26 which is controlled by a
coloring beam control signal, a collimator lens 25, an optical element 24
for controlling the shape of the coloring beam spot, and the dichroic
mirror 29. This coloring beam spot is applied to the optical disk 20 to
precede a reproducing beam spot. The reproducing beam spot is applied by
condensing a beam which is emitted from the semiconductor laser 32 on the
optical recording medium 20 through the objective lens 23, similarly to
the information recording. The coloring beam spot is adjusted by the
optical element 24 to have a spot size which is larger than that of the
reproducing beam spot. The recording/reproducing beam spot is
substantially in the form of a complete round having a spot size of 1.3
.mu.m, while the coloring beam spot is formed to have a spot size of 2.2
.mu.m.
The reproducing beam spot which is reflected through the recording layer of
the optical disk 20 passes through the dichroic mirror 29 and is
transferred to a servo/signal detection optical system 33 by the polarized
beam splitter 30, so that a reproducing signal is taken out. A servo
signal is transmitted to a control circuit 34, which in turn transmits a
control signal for adjusting the objective lens 23.
The aforementioned recording/reproducing apparatus was employed in
practice, to record signals having frequencies of 300 kHz to 6 MHz in the
aforementioned respective optical recording media with semiconductor laser
beams of 670 nm in wavelength when the masking layers were in decolored
states with large transmittance values in a magnetic field modulation
manner with recording power of 7 mW and relative speeds of 5.5 m/sec. The
recorded signals were reproduced with reproducing beam power of 6 mW and
coloring beam power of 5 mW, for measuring relations between frequencies
and reproduction outputs.
FIG. 10 illustrates relations between frequency characteristics and outputs
which were measured in the aforementioned manner. With reference to 0 dB
at a frequency of 300 kHz, the outputs are reduced by 6 dB at frequencies
of about 4.7 MHz, about 4.5 MHz and 5.8 MHz in comparative examples 1 and
2 and Example respectively, as shown in FIG. 10. Thus, it is understood
that linear recording density was remarkably improved in the optical
recording medium according to Example. It is possible to improve a
super-resolution effect by providing a masking layer which is facilitated
in photon mode photochromic reaction by temperature rise, thereby
improving linear recording density. It is also possible to improve track
density, in addition to the linear recording density.
While a diarylethene photochromic material is employed in the
aforementioned embodiment, the material for the masking layer is not
restricted to this but any material can be employed so far as the same
causes decoloration by a photon mode photochromic reaction which is
facilitated by temperature rise. For example, it is possible to employ a
fulgide-based compound having the following structure, which is described
in Nippon Kagaku Kaishi, No. 8 (1985), p. 1598, as the material for the
masking layer according to the present invention:
##STR6##
FIG. 6 illustrates absorption spectra of the fulgide-based compound having
the aforementioned structure. Curves (i) and (ii) show absorption spectra
in decolored and colored states respectively. In this fulgide-based
compound, the quantum yield of a photon mode photochromic reaction which
is an decoloration reaction is increased with the temperature. Therefore,
the photon mode photochromic reaction which is the decoloration reaction
is facilitated by temperature rise, to abruptly progress at a prescribed
temperature with nonlinearity. Therefore, it is also possible to employ
this material for the masking layer according to the present invention.
While the recording layer is made of a magneto-optical recording material
in the aforementioned embodiment, the present invention is not restricted
to this but is also applicable to a bit formation recording medium such as
a compact disk and a phase change type medium, while the same is also
readily applicable to a short-wavelength laser and mark edge recording.
The coloring laser beam source can also be formed by an SHG laser or a blue
semiconductor laser, which is expected to be put into practice in the near
future, if absorption characteristics of the masking layer are optimized.
Such lasers are also employable for the reproducing beam source and lasers
having a longer wavelength are employable for the coloring beam source.
While the coloring beam spot and the reproducing beam spot are condensed on
the optical recording medium by the same objective lens in the above
embodiment, it is also possible to employ different optical systems for
condensing these beams on the optical recording medium respectively.
According to the present invention, the masking layer is converted to a
state having small absorption at the wavelength of the reproducing beam by
a thermal reaction or a photon mode photochromic reaction which is
facilitated by temperature rise upon irradiation with the reproducing
beam. Decoloration of such a masking layer abruptly progresses as a
temperature increases, whereby high nonlinearity can be attained.
Therefore, it is possible to attain a super-resolution effect by providing
such a masking layer for reducing an effective spot size for reproduction
as compared with the reproducing beam spot, thereby enabling
recording/reproduction of information in high density.
The masking layer can be decolored by beam application with temperature
rise by heating, to be optically transparent. Thus, the inventive optical
recording medium can be handled similarly to a medium which is provided
with no masking layer, to maintain compatibility.
Although the present invention has been described and illustrated in
detail, it is clearly understood that the same is by way of illustration
and example only and is not to be taken by way of limitation, the spirit
and scope of the present invention being limited only by the terms of the
appended claims.
* * * * *
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