JPH10147068A - Optical recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform a high density recording by forming a thermochromic layer provided on a light transmission board so as to have properties of increasing transmittance due to decrease of absorbance at a temperature higher than a threshold value and increasing the absorbance to return to the original state when it is cooled to the temperature lower than the threshold value. SOLUTION: An optical disk 1 as the optical recording medium is constituted by sequentially laminating a thermochromic layer 3, a reflecting layer 4 and a protective layer 5 on a resin board 2. The layer 3 is formed to have reversible properties of lowering its transmittance at a temperature or lower than a threshold value due to large absorbance and raising it at a temperature or higher than the threshold value due to reduced absorbance and returning to an original state due to the increase of the absorbance when it is cooled further to the temperature or lower than the value. According to this, at the time of emitting a spot of a laser beam, the light transmitted through the layer becomes smaller effective spot size than that of the emitted light, and hence high density information can be recorded and reproduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical recording medium capable of high-density recording, and more particularly to an optical recording medium having a layer containing a thermochromic material whose transmittance changes according to the intensity of irradiation light. The present invention relates to an optical recording medium capable of performing high-density recording and reproduction by reducing the effective spot diameter.

[0002]

2. Description of the Related Art In recent years, studies have been made on increasing the capacity of optical recording media such as optical disks, and various proposals have been made. For example, studies have been made on reducing the track pitch for increasing the capacity, shortening the wavelength of laser light used for recording and reproduction, and increasing the numerical aperture of an objective lens. In recent years, as a short-wavelength laser beam used for reading / writing information from / to a disc, a SH using a nonlinear optical element has been used.
G (Second Harmonic Generator)
or) is being considered. For example, it has been studied to extract and use the second harmonic light of 400 nanometers from an 800 nanometer laser light using an SHG element. However, practical use is required in terms of conversion efficiency, price, stability, and the like. At present, it has not yet reached a level where it can be offered. In addition, the short wavelength 400 to 5
The semiconductor laser of blue-green to blue-violet light of 00 nanometers has also been actively researched, but it is at the stage where room temperature oscillation has recently become possible, and it can be put to practical use in terms of output and stability. It is not yet at the level.

[0003] Semiconductor laser light widely used as a currently practical laser has a limit of about 690 to 635 nm. Incorporating a high numerical aperture lens for an optical disc requires a numerical aperture of at most NA because of problems such as depth of focus and strict physical accuracy (eg, thickness, warpage, surface runout, etc.) of the disc. Is 0.6. Even if a laser beam having a wavelength of 690 to 635 nanometers is used and a lens having a numerical aperture NA of 0.6 is used, the recording density can be improved to about four times that of a CD (compact disk). It is.

Accordingly, with respect to a technology for recording and reproducing information at a higher density, by reducing the effective irradiation spot diameter of an optical information recording medium, high-density optical information that could not be recorded and reproduced until now has been reduced. Various researches and developments have also been made on recording / reproducing (so-called disk super-resolution). As one example, Japanese Patent Application Laid-Open No. 7-182693 discloses an optical recording medium which realizes super resolution of a disk using a thermochromic substance.
And Japanese Patent Application Laid-Open No. 7-311978. In this technology, when the laser beam is irradiated, only the central portion of the irradiated portion of the thermochromic layer becomes partially light-transmissive due to a rise in temperature. This makes it possible to perform high-density recording / reproduction that could not be performed until now.

[0005]

However, in an optical recording medium which realizes super-resolution of a disk by using a thermochromic substance, the same place (the same track) is repeatedly irradiated with a laser beam while rotating the disk to reproduce. When the so-called still reproduction is performed, the thermochromic substance whose transmittance changes depending on the temperature is repeatedly irradiated with laser light before completely returning to the original transmittance (colored state). For this reason, when the number of times of still reproduction in which laser light is continuously and repeatedly applied increases, the transmittance does not return to the original value, so that the size of the light transmitting portion increases and the effective spot diameter decreases accordingly. There is a problem that the effect (mask effect) decreases.
The present invention has been made to solve the above problems, and an object of the present invention is to irradiate light so that only a central portion of an irradiated portion partially transmits light due to a temperature rise. Utilizing the fact that the effective irradiation spot diameter is reduced, thereby making it possible to record / reproduce a high-density recorded medium that could not be recorded / reproduced, and An object of the present invention is to provide an optical recording medium having a sufficient stability with respect to continuous and repetitive still reproduction.

[0006]

Means for Solving the Problems The present inventor made an optical disc having various thermochromic layers and studied the still reproduction characteristics for repeatedly reproducing the same track. As a result, the thermochromic layer at the laser wavelength to be irradiated was obtained. Absorbance of the thermochromic layer significantly affects the still reproduction characteristics (that is, heat storage generated by the laser light repeatedly irradiated to the same place), and the absorbance of the thermochromic layer at the irradiated laser wavelength is 1.2 abs or more.
5abs or less, preferably 1.5abs or more and 2.2a
At bs or less, the present invention has been achieved by finding that the effective spot diameter is kept small and sufficient stability is obtained even when continuous still reproduction is performed.

According to the present invention, there is provided an optical recording medium having at least a thermochromic layer and a reflective layer provided on a light-transmitting substrate, wherein the thermochromic layer has a wavelength with respect to a wavelength of a laser beam used for optical recording and reproduction. Absorbs at temperatures below the threshold, absorbance decreases at temperatures above the threshold to increase transmittance, and further cools to temperatures below the threshold to increase absorbance and return to the original state. In addition, the absorbance before the laser light irradiation is set to be in a range of 1.2 abs to 2.5 abs with respect to the wavelength of the laser light. Thereby, even if still reproduction is performed continuously, the effective irradiation spot diameter does not increase, the mask effect can be maintained high, and good still reproduction characteristics can be obtained. In particular, by providing a heat-dissipating layer made of a transparent inorganic substance between the light-transmitting substrate and the thermochromic layer, or between the thermochromic layer and the reflecting layer, or both, the heat-dissipating efficiency of the thermochromic layer is increased. be able to. Therefore, in this case, the still characteristics can be kept higher.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the optical recording medium according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a partially enlarged sectional view showing an optical recording medium according to the present invention. In the present embodiment, a case where the present invention is applied to an optical disc as an optical recording medium will be described as an example. In FIG. 1, an optical disk 1 as an optical recording medium has a light-transmitting resin substrate 2 having pits and grooves formed on its surface, on which a thermochromic layer 3, a reflective layer 4 made of a metal film and The protective layer 5 is sequentially laminated. In the figure, arrows indicate the directions in which laser light used for recording and reproduction is irradiated.

The optical recording medium 1 according to the present invention is applicable to both a read-only type and a recording / reproducing type. As the light-transmitting resin substrate 2, a resin that is usually used as a substrate of an optical disk, such as a polycarbonate resin, a polymethacrylate resin, or an epoxy resin, can be used. On the resin substrate 2, pits are formed in the case of the read-only type, and grooves are formed in the case of recordable to form a recording surface. There is no particular limitation on the method of forming the pits and grooves, and the pits and grooves are formed by an ordinary method.

A thermochromic layer 3 is provided on a light-transmitting resin substrate 2. The thermochromic layer 3 is made of a thermochromic substance, and has a large absorbance at a temperature lower than the threshold value with respect to the wavelength of the laser beam used for optical recording / reproducing, so that the transmittance is low and an absorbance value at a temperature higher than the threshold value. Decreases, the transmittance increases, and when the temperature is further cooled to a temperature lower than the threshold value, the absorbance increases again, the transmittance decreases, and the film returns to its original state, which is a reversible property. On the other hand, if the thermochromic layer 3 absorbs the wavelength of the laser used, the temperature of the thermochromic layer 3 increases due to light irradiation. That is, if the change in the transmittance of the thermochromic layer with respect to the temperature has the characteristics shown in FIG. 2, only the portion heated to a temperature equal to or higher than the transmittance change point P1 has a higher transmittance, and the other portions have the absorbance. And the transmittance remains low. Here, the temperature is substantially proportional to the light intensity, and the horizontal axis is represented as the light intensity in FIG. The light intensity at the transmittance change point P1 is represented by Is. Thus, an effect of substantially reducing the diameter of the irradiated light spot can be exhibited.

When such a thermochromic layer 3 is provided, when a light spot of a laser beam having a normal distribution of intensity distribution as shown in FIG. 3 is irradiated, light transmitted through this layer is shown in FIG. Like a light spot described in correspondence with the intensity distribution curve, the effective spot diameter L2 is smaller than the irradiation spot diameter L1, resulting in a substantially reduced spot.
Therefore, recording and reproduction of high-density information can be performed. The effective irradiation spot diameter L2 at this time depends on the light intensity Is at the transmittance change point P1. As the thermochromic substance, various substances having the above properties can be used.For example, an electron donating color compound and an electron accepting developer, a mixed system of a polar compound or an electron donating color compound may be used. Mixtures of phenolic color developing materials are included. Examples of the electron donating color compound include a fluoran compound, a spiropyran compound, a phthalide compound, and a lactam compound.

The reflection layer 4 is similar to a metal reflection layer generally used in an optical disk, and is formed of a thin film of a metal or alloy such as gold or aluminum. A protective layer 5 may be provided on the reflective layer 4 as needed for the purpose of protecting the medium. This protective layer 5 can be easily formed by providing an ultraviolet curable resin by a spin coating method. Here, in the present invention, with respect to the thermochromic layer 3, the absorbance of the thermochromic layer 3 at the wavelength of the irradiated laser beam is 1.2 abs to 2.5 abs.
abs, preferably 1.5 abs to 2.2 ab
s is set within the range. This allows
Even when continuous still reproduction was performed, the effective spot diameter could be kept small without reducing the mask effect, and sufficient stability could be obtained.

That is, the thermochromic substance used for the thermochromic layer 3 has a colored state having absorption in the wavelength region of the laser light to be irradiated when the temperature is low, and a wavelength region of the laser light when the temperature is high. It is a substance that changes reversibly between a transparent state and no absorption. The temperature rises when irradiated with laser light, the thermochromic substance that has changed from a colored state to a transparent state, the laser light irradiation stops, the temperature decreases, and a reversible return to the original colored state takes a certain amount of time. Takes time. In the case of still playback of an optical disk, the time required to make one round around the same track is about several milliseconds to several tens of milliseconds, and the thermochromic layer heated by laser irradiation within the short time returns to the original temperature. I can't go back. That is, in the still reproduction, the disk rotates once while heat is accumulated and the next laser beam is irradiated. Therefore, the temperature of the thermochromic layer gradually increases during the still reproduction, and as a result, the light The size of the transmissive portion gradually increases, and the effect (mask effect) of reducing the effective irradiation spot diameter L2 decreases. This mask effect appears even when the absorbance of the thermochromic layer is smaller than 1.2 abs. However, in still reproduction in which the same track is repeatedly reproduced, the frequency characteristic (f-characteristic) rapidly deteriorates as the number of repetitions increases. Become.

On the other hand, when the absorbance of the thermochromic layer is 1.2 abs or more as in the present invention, the frequency characteristic (f characteristic) does not change at all even if the number of times of still increases, or the frequency characteristic deteriorates. Can be significantly reduced. This is considered to be because the large absorbance of the thermochromic layer has the effect of suppressing the spread of the effective spot diameter L2 due to repetition of still reproduction. If the light absorption of the thermochromic layer 3 is too large, the irradiated laser light is almost absorbed, so that a considerable laser power is required to obtain the reflected light, and the film thickness of the thermochromic layer 3 is further increased. Therefore, it becomes difficult to follow the shape of pits or grooves formed on the light-transmitting resin substrate, and tracking during reproduction of the disc becomes unstable. Therefore, the absorbance of the thermochromic layer must be 2.5 abs or less.

That is, as a result of forming various disks and conducting experiments, the absorbance of the thermochromic layer 3 was 1.
If it is smaller than 2 abs, the frequency characteristic rapidly deteriorates as the number of repetitions of the still reproduction increases, and if the absorbance is larger than 2.5 abs, the reflected light cannot be sufficiently detected, and tracking becomes unstable. I have. Therefore, the absorbance of the thermochromic layer 3 is 1.2a
bs to 2.5 abs. In particular, setting the absorbance in the range of 1.5 abs to 2.2 abs was the most preferable in terms of characteristics.

Further, as shown in FIG. 4, heat radiation layers 6 and 7 made of a transparent inorganic substance are formed between the light-transmitting resin substrate 2 and the thermochromic layer 3 or between the thermochromic layer 3 and the reflection layer 4. By providing heat radiation layers 6 and 7 made of a transparent inorganic substance both between the light transmissive substrate and the thermochromic layer 3 and between the thermochromic layer 3 and the reflection layer 4, the thermochromic layer is provided. 3 can be efficiently radiated, thereby preventing a reduction in the mask effect and further suppressing the deterioration of the frequency characteristics. The illustrated example shows a case where two heat radiation layers 6 and 7 are provided. In the figure, arrows indicate the directions in which laser light used for recording and reproduction is irradiated.
The heat radiation layers 6 and 7 made of a transparent inorganic material are preferably formed of any one of a metal oxide, a non-metal oxide, a metal halide, and a metal sulfide, such as silicon dioxide, cerium oxide, and magnesium fluoride. And zinc sulfide. It is preferable that these heat radiation layers 6 and 7 have higher thermal conductivity than the material of the resin substrate 2, so that the heat accumulated in the thermochromic layer 3 can be released more quickly.

Next, examples of the optical disk of the present invention actually formed and comparative examples will be described. <Example 1> Example 1 of the present invention using GN-169 (manufactured by Yamamoto Kasei) as a thermochromic layer and using bisphenol A as a color developing material will be described. At a density four times that of a compact disk (CD), a polycarbonate resin substrate in which signals are provided as minute pits is injection-formed, and GN-169 (Yamamoto Kasei) as an electron-donating color compound is formed thereon. Bisphenol A was formed as a color developing material by a vacuum co-evaporation method.
A thermochromic layer having a thickness of 00 nm was formed. This thermochromic layer was treated with an ultraviolet-visible spectrophotometer (UV-PC3101 manufactured by Shimadzu Corporation) at a wavelength of 680.
The absorbance at the nanometer is 1.87ab
s. Next, aluminum was applied as a reflective layer on the thermochromic layer by vacuum sputtering to about 7 μm.
It was formed to a thickness of 0 nanometer. Further, a UV-curable resin SD-17 (manufactured by Dainippon Ink) was formed as a protective layer to a thickness of about 7 μm to produce an optical disk.

When the above optical disc was reproduced by a player equipped with a semiconductor laser having a wavelength of 680 nm, the ratio of the reproduction amplitude of the shortest pit to the reproduction amplitude of the longest pit was 83% at the beginning as shown in FIG. Was still 70% even after 5000 times of still reproduction, and good results could be obtained. The reproduction conditions at this time are as follows: the linear velocity CLV is 3 m / s, and the rotation speed is 1000 r.
pm and reproduction power were about 3.0 mW.

<Embodiment 2> Next, Embodiment 2 in which a heat radiation layer made of a transparent inorganic substance is provided between a light-transmitting resin substrate and a thermochromic layer will be described. Injection molding a polycarbonate resin substrate provided with pits with minute signals at 4 times the density of a compact disk, and forming magnesium fluoride as an inorganic substance on the substrate by vacuum evaporation to form a 25 nm thick heat dissipation layer And On this, GN-169 (manufactured by Yamamoto Kasei) as an electron donating color compound and bisphenol A as a color developing material were formed into a film by vacuum co-evaporation method. A thermochromic layer having a thickness of 900 nanometers was formed. The absorbance at a wavelength of 680 nanometers was 1.83 abs. Less than,
A reflective film and a protective film were formed in the same manner as in Example 1, and an optical disk was manufactured. When the above optical disk was reproduced by a player equipped with a semiconductor laser having a wavelength of 680 nm, the ratio of the reproduction amplitude of the shortest pit to the reproduction amplitude of the longest pit was 81% at the beginning as shown in FIG. Even after the still image has been reproduced 5000 times,
8%, which was better than Example 1. The reproduction condition at this time is that the linear velocity CLV is 3 m /
s, rotation speed 1000 rpm, reproduction power about 3.0 m
W.

<Comparative Example 1> Next, as Comparative Example 1, a polycarbonate resin substrate used as in Example 1 and having a density four times that of the compact disc and provided with pits having minute signals is injection-molded. An optical disk was produced by directly forming a reflective film and a protective layer in the same manner as in Example 1 without providing a thermochromic layer thereon. When the above optical disc was reproduced by a player equipped with a semiconductor laser having a wavelength of 680 nm, there was no effect of reducing the spot diameter of the irradiation laser beam, and as shown in FIG. And the ratio of the reproduction amplitude of the longest pit was 40%, which was an undesirable characteristic. The reproduction condition at this time is the linear velocity CLV.
Was 3 m / s, the rotation speed was 1000 rpm, and the reproducing power was about 1.0 mW.

<Comparative Example 2> Next, as Comparative Example 2, a polycarbonate resin substrate provided with fine pits with signals at a density four times that of the compact disk used in Example 1 was injection-molded. A film of GN-169 (manufactured by Yamamoto Kasei) as an electron donating color compound and a film of bisphenol A as a color developing material were formed on the monitor by a vacuum co-evaporation method. Is 5
A thermocrack layer having a thickness of 00 nm was formed. The measured absorbance at a wavelength of 680 nm was 0.89 abs. Hereinafter, a reflective film and a protective film were formed in the same manner as in Example 1 to produce an optical disc.

When the above optical disc was reproduced by a player equipped with a semiconductor laser having a wavelength of 680 nm, the ratio of the reproduction amplitude of the shortest pit to the reproduction amplitude of the longest pit was 81% at the beginning as shown in FIG. Was deteriorated to 40% after 5000 times of still reproduction, which was an undesirable characteristic. The reproduction conditions at this time are as follows: the linear velocity CLV is 3 m / s, and the rotation speed is 100.
At 0 rpm, the reproducing power was about 3.0 mW.

[0023]

As described above, according to the optical recording medium of the present invention, the following excellent functions and effects can be exhibited. Absorbance of 1.2a for recording / reproducing laser light
By using a thermochromic layer having a range from bs to 2.5 abs, the recording / reproducing spot diameter (effective irradiation spot diameter) can be substantially reduced, high-density recording / reproducing is possible, and the same track for still reproduction is used. , The signal characteristics are not significantly degraded even when the reproduction is repeatedly performed, and it is possible to stably perform good signal reproduction. Further, by providing a heat dissipation layer made of a transparent inorganic substance between the resin substrate and the thermochromic layer or between the thermochromic layer and the reflection layer, it is possible to further suppress deterioration of signal characteristics during still reproduction.

[Brief description of the drawings]

FIG. 1 is a partially enlarged sectional view showing an optical recording medium according to the present invention.

FIG. 2 is a graph showing a change in transmittance of a thermochromic layer with respect to light intensity (temperature).

FIG. 3 is an explanatory diagram for explaining a change in a spot diameter of irradiation laser light.

FIG. 4 is a partially enlarged sectional view showing another embodiment of the present invention.

FIG. 5 is a graph showing evaluations of examples of the present invention and comparative examples.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Optical disk, 2 ... Resin substrate (light transmission substrate), 3 ...
Thermochromic layer, 4 ... reflective layer, 5 ... protective layer, 6,7
... heat radiation layer, L1 ... irradiation spot diameter, L2 ... effective irradiation spot diameter.

Claims (2)

[Claims] 1. An optical recording medium having at least a thermochromic layer and a reflective layer on a light-transmitting substrate,
The thermochromic layer has an absorption at a temperature lower than the threshold with respect to the wavelength of the laser beam used for optical recording and reproduction, and at a temperature higher than the threshold, the absorbance decreases and the transmittance increases, further lower than the threshold. When cooled to a temperature, it has a property that the absorbance increases and returns to the original state, and the absorbance before the laser light irradiation is 1.2ab with respect to the wavelength of the laser light.
s to 2.5 abs. 2. A heat radiation layer made of a transparent inorganic substance is provided between the light transmissive substrate and the thermochromic layer and / or between the thermochromic layer and the reflection layer. 2. The optical recording medium according to 1.