JP2844824B2 - Optical disk signal reproduction method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal reproducing method for an optical disk having phase pits formed according to an information signal.

[Summary of the Invention]

According to the present invention, the reflectivity (spectral characteristic) of an optical disk having phase pits formed in accordance with a signal is changed with temperature, and the reflectivity is partially changed within a scanning spot of a readout light. This enables reproduction of high-density information.

[Conventional technology]

For example, in an optical disk such as a digital audio disk (a so-called compact disk) or a video disk, a phase pit is formed in advance on an information recording surface in accordance with a signal, and when a scanning spot of a reading light is applied to the phase pit. The signal is reproduced by utilizing the fact that the amount of reflected light is greatly reduced due to diffraction, and detecting the decrease in the amount of reflected light with a photodetector.

By the way, the signal reproduction resolution of an optical disk is almost the same as the wavelength λ of the light source of the reproduction optical system and the numerical aperture of the objective lens.
Determined by NA, the spatial frequency 2NA / λ is the reading limit.

Therefore, in order to realize high density in an optical disc, it is necessary to shorten the wavelength λ of the light source (for example, a semiconductor laser) of the reproducing optical system and increase the numerical aperture NA of the objective lens.

[Problems to be solved by the invention]

However, there is a natural limit in improving the wavelength of the light source and the numerical aperture of the objective lens, and it is difficult to dramatically increase the recording density.

Accordingly, the present invention has been proposed in view of such a conventional situation, and has been proposed in order to reproduce a high-density phase pit information exceeding a reading limit based on a wavelength of a light source and a numerical aperture of an objective lens. It is intended to provide a reproduction method.

[Means for solving the problem]

According to the signal reproducing method of the present invention, a reading light is applied to an optical disk in which a thin film layer whose reflectivity changes with temperature is formed on a recording surface on which phase pits corresponding to signals are formed. Then, the reflectance of the thin film layer is partially changed in the scanning spot of the reading light due to a temperature rise caused by irradiating the reading light to the optical disk. Then, the phase pits in the scanning spot are read while the reflectance of the thin film layer is partially changed in the scanning spot of the reading light.

[Action]

When reading light is applied to an optical disk in which a thin film layer whose reflectance changes with temperature is formed on a recording surface on which phase pits corresponding to signals are formed, a temperature distribution occurs in a scanning spot of the reading light, For example, the reflectance of a portion having a temperature higher than a certain value is improved.

As a result, the remaining portion of the scanning spot has a so-called masked shape, and only the phase pits of the portion where the reflectance is improved are read.

That is, as shown in FIG. 1, a plurality of phase pits (2) out of the phase pits (2) formed in accordance with the signal.
When a) and (2b) enter the scanning spot (1) of the readout light, only the phase pits (2a) in the high reflectivity region (3) whose reflectivity has improved due to the temperature rise are detected.
On the other hand, in the area other than the high reflectance area (3) (the shaded area in the figure), the reflectance is low regardless of the presence or absence of the phase pit (2). ), The change in reflectance in the scanning spot (1) due to this is small, and is not detected as a signal.

〔Example〕

Hereinafter, specific examples to which the present invention is applied will be described.

Embodiment 1 This embodiment is an example using an optical disc whose reflectivity changes due to a phase change.

FIG. 2 shows the structure of the optical disk used in this embodiment. This optical disk has a first silicon nitride film (12) on a transparent substrate (11) on which phase pits are formed in accordance with an information signal. ), A Sb ₂ Se ₃ film (13), a second silicon nitride film (14), and an Al film (15) are sequentially laminated.

The Sb ₂ Se ₃ film (13) is a chalcogen-based amorphous material, and has a large difference in reflectance between the amorphous state and the crystalline state. For example, the first silicon nitride film (1
2) The thickness of the second silicon nitride film (14) is set to 1000 Å, the thickness of the second silicon nitride film (14) is set to 500 、, and the thickness of the Sb ₂ Se ₃ film (13) is changed. As shown in FIG. 3, the reflectance in the amorphous state is higher than that in the amorphous state. In particular, when the thickness of the Sb ₂ Se ₃ film (13) is 900 °, the reflectance in the amorphous state is about 60%. %, And the reflectance in the crystalline state is 1% or less.

Therefore, in this embodiment, high-density reproduction is performed by utilizing the difference between the reflectances in the amorphous state and the crystalline state. Hereinafter, the reproducing method of this embodiment will be described.

In the above-described optical disc, the Sb ₂ Se ₃ film (13) is made to be in a crystalline state over its entire surface, and is in an initialized state.

When a laser beam is irradiated as read light to the optical disk in the initialized state, a part of the laser spot becomes amorphous and the reflectance increases. This is the fourth
Shown in the figure.

When the optical disk is irradiated with the laser spot (21),
The light intensity in the laser spot (21) shows a distribution as shown by a broken line A in the figure, and the temperature distribution of the Sb ₂ Se ₃ film (13) is slightly delayed corresponding to the scanning speed of the laser spot (21). It will be. (Curve B in FIG. 4) Here, assuming that the laser spot (21) is scanned in the direction of the arrow X in FIG. 4, the temperature gradually increases from the tip side in the running direction of the laser spot (21), and finally. Is higher than the melting point T of the Sb ₂ Se ₃ film (13). At this stage, Sb ₂ Se ₃
The film (13) changes from a crystalline state to a liquid state. When the laser spot (21) passes, plummeted temperature of the Al film (15) above Sb ₂ Se ₃ film by the cooling effect of (13), only the part exceeding the melting point T from the liquid state to the amorphous state And the reflectivity rises significantly. The region which has been made amorphous as described above is indicated by region P in the figure.
A line C in FIG. 4 indicates the reflectance of the Sb ₂ Se ₃ film (13) along the locus of the center point of the laser spot (21).

Since the Sb ₂ Se ₃ film (13) before being heated to the melting point T by irradiation of the laser spot (21) is in a crystalline state, the reflectance is extremely low, and it is assumed that the phase pit (22) exists in this region. However, the change in reflectance due to its presence is slight,
It is not extracted as a signal. On the other hand, in the region P heated to the melting point T and amorphized by the irradiation of the laser spot (21), the Sb ₂ Se ₃ film (13) shows a high reflectance, and a change in the reflectance due to the presence or absence of the phase pit (22) is a signal. Is taken out as

That is, when viewed in the laser spot (21),
Portion before being heated to the melting point T is substantially longer participate in the reproduction of the signal, the phase pit (22) only in the amorphized region P _X shown in hatched area is detected. Therefore, a part of the laser spot (21) is masked, so that a window is opened in the laser spot (21) so that the diameter of the laser spot (21) is reduced equivalently.
High-density reproduction, which greatly exceeds the detection limit determined by the numerical aperture of the objective lens and the wavelength of the semiconductor laser, becomes possible.

Note that actually the magnitude of the phase pit area for reading a signal by (22) (amorphized region) P _X shall be Kotororu appropriately by the upper and lower laser power [temperature and below the That Sb ₂ Se ₃ film (13)] Is possible.

Further, according to the above method, since the amorphous state remains on the optical disk as a stable state, energy is applied by some method, and after the reproduction, an erasing operation for returning the amorphous state to the original crystalline state is performed. Is preferred. For example, by irradiating an elliptical spot after a laser spot for reproduction and heating the Sb ₂ Se ₃ film (13) to a temperature lower than the melting point T and higher than the crystal and temperature, the Sb ₂ Se ₃ film (13) becomes amorphous. Transition from the state to the crystalline state.

As described above, the embodiment in which high-density reproduction is performed by using a change in reflectivity due to a phase change using a chalcogen-based amorphous material has been described. However, the chalcogen-based amorphous material is not limited to the Sb ₂ Se ₃ described above. For example, Te-Ge
-Sn-O-based amorphous materials, In-Se-based amorphous materials, In-Sb-based amorphous materials, and the like can be used, and amorphous materials other than chalcogen-based materials may be used.

Embodiment 2 This embodiment utilizes a change in spectral characteristics due to moisture adsorption in an interference filter.

The structure of the optical disk used in this embodiment is as shown in FIG. 5, and the transparent substrate (3
1) An interference filter is formed by repeatedly forming a material having a significantly different refractive index on the upper surface so that the thickness becomes 1/4 of the wavelength of the reproduction light.
In this example, as a material having a significantly different refractive index, the MgF layer (3
2) [Refractive index 1.38] and ZnS layer (33) [refractive index 2.35] were adopted. Of course, not limited to this, any combination of materials having significantly different refractive indices may be used. For example, a material having a small refractive index includes SiO (refractive index 1.5) and the like, and a material having a large refractive index. TiO ₂ [refractive index
2.73] and CeO ₂ [refractive index 2.35].

Although MgF layer (32) and ZnS layers above (33) is deposited forming the ultimate degree of vacuum in forming these evaporation example ^10-4
If the pressure is set to about Torr, which is lower than usual, the film structure becomes so-called porous, and water remains therein. Then, in the interference filter composed of the film in which the water remains, the temperature is raised between room temperature and the temperature near the boiling point of water.
For example, as shown in FIG. 6, the reflectance spectral characteristics are significantly different. That is, at room temperature, the characteristic that the wavelength λ _R is an inflection point as shown by the curve i in the figure, whereas when the temperature is raised to near the boiling point, the wavelength λ _H inflections as shown by the curve ii in the figure. A steep wavelength shift is observed such that the characteristic becomes a point, and returns to the characteristic shown by the curve i when the temperature decreases. This development is considered to be due to the fact that the vaporization of water significantly changes the rate of segregation, and this effect changes the spectral characteristics.

Therefore, the wavelength of the light source of the reproduction light is changed to these inflection points λ _R ,
If the wavelength λ ₀ is selected as an intermediate wavelength of λ _H , the reflectance changes dynamically between room temperature and heating.

In this embodiment, high-density reproduction is performed using this change in reflectance. The principle that enables high-density reproduction is as shown in FIG. 1. In this case, the region where wavelength shift occurs due to vaporization of water corresponds to the high reflectance region (3), and the temperature rises. The parts that are not covered are masked. However, in this example, when the temperature drops, the reflectance characteristic returns to the original state, so that no special erasing operation is required.

As described above, specific examples of the present invention have been described.
The present invention is not limited to these embodiments, and the change in the reflectivity may use any phenomenon.
When reproducing, a sharp temperature change can be given by using a pulse light source, and the portion where the reflectance changes can be made sharper.

〔The invention's effect〕

As is clear from the above description, in the present invention, the reflectivity (spectral characteristic) of the optical disk is changed according to the temperature, and the reflectivity is partially changed in the scanning spot of the reading light. It is possible to mask a part of the scanning spot and reduce the scanning spot diameter equivalently, thereby enabling high-density reproduction that greatly exceeds the detection limit determined by the numerical aperture of the objective lens and the wavelength of the semiconductor laser. .

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining the principle of reproduction in the present invention. FIG. 2 is a schematic cross-sectional view of a main part showing an example of the configuration of an optical disk utilizing a change in reflectivity due to a phase change, and FIG. 3 shows Sb ₂ Se ₃
FIG. 4 is a characteristic diagram showing the dependence of the reflectance on the film thickness in the amorphous state and the crystalline state, and FIG. 4 is a schematic diagram showing the phase change and the reflectance change by scanning the laser spot together with the temperature distribution of the laser spot. FIG. 5 is a schematic cross-sectional view of a main part showing an example of the configuration of an optical disk utilizing a change in reflectance spectral characteristics of an interference filter containing moisture, and FIG. 6 shows changes in reflectance spectral characteristics of an interference filter with temperature. It is a characteristic view showing a situation.

Claims (1)

(57) [Claims] An optical disk comprising a recording surface on which phase pits corresponding to a signal are formed and a thin film layer whose reflectance changes with temperature is formed is irradiated with read light, and the read light is irradiated on the optical disk. As a result, the reflectance of the thin film layer is partially changed within the scanning spot of the readout light, and the reflectance of the thin film layer is partially changed within the scan spot of the readout light. A signal reproducing method for an optical disk, characterized by reading phase pits in the optical disk.