HK1062351B - Optical disk - Google Patents

Description

Optical disk

Technical Field

The present invention relates to an optical disc having a light-transmitting layer through which a data signal is recorded on the optical disc and from which the recorded data signal is reproduced.

Background

The optical disks used so far each include a recording layer and a reflective layer, etc., which are laminated on each other on a substrate. A light beam is applied to a recording layer of the optical disc, thereby recording a data signal on the optical disc. Another light beam is applied to the recording layer, thereby reproducing the data signal recorded on the recording layer. It is required to increase the recording density of optical discs so that as much data as possible can be recorded on each optical disc. Various techniques have been proposed to meet such a demand.

An increase in the recording density of an optical disc, i.e., the storage capacity per disc, can be achieved by some methods. One method is to shorten the wavelength of a light beam applied from a light source for recording a data signal onto an optical disk and the wavelength of a light beam applied for reproducing a data signal from an optical disk. Another method is to increase the Numerical Aperture (NA) of an objective lens for focusing a light beam emitted from a light source onto a signal recording layer of an optical disc, thereby reducing the size of a spot formed on a focal plane.

For a CD (compact disc, a type of optical disc), for example, a light beam having a wavelength of 780nm is applied to read a data signal from the CD, and an objective lens having a Numerical Aperture (NA) of 0.45 is employed. Thereby increasing the storage capacity of each CD to 650 MB. For DVD-ROMs (digital versatile disc read only memory), the storage capacity of each DVD-ROM is increased to 4.7GB by applying a light beam having a wavelength of 650nm and using an objective lens having a Numerical Aperture (NA) of 0.6.

To this end, the applicant proposed an optical disc: when a short-wavelength light beam is applied through an objective lens having a large Numerical Aperture (NA), data can be recorded at high density. The optical disc has a transparent layer or a thin cover layer. The light beam is first applied to the light-transmitting layer, thereby recording data on the optical disc or reproducing data from the optical disc. More precisely, a light beam having a wavelength shorter than 450nm is applied to the optical disc through an objective lens having a Numerical Aperture (NA) larger than 0.78. The optical disc can store 22GB or more of data.

When a light beam is applied to any one of the optical disks to record a signal thereon or reproduce a signal therefrom, it is necessary to minimize spherical aberration occurring when the light beam is focused on the optical disk. For this purpose, the objective lens and the optical disc are designed according to their optical characteristics.

More specifically, the refractive index and thickness of a light-transmitting layer of an optical disc are optimized according to the optical characteristics of an objective lens that focuses a light beam onto the optical disc. This measure minimizes the spherical aberration that occurs when a light beam is applied to the optical disc. In other words, the refractive index and thickness of the light-transmitting layer are set to values that minimize the spherical aberration caused by the objective lens.

An optical disc having a recording layer and a light transmitting layer covering the recording layer, to which a short-wavelength light beam is applied through an objective lens having a large Numerical Aperture (NA), to record data at high density and reproduce data, is considered. Because of the large Numerical Aperture (NA) of the objective lens, any light beam focused on the recording layer of the optical disc will have spherical aberration if the refractive index of the light-transmitting layer is different from the ideal refractive index of the medium that will minimize spherical aberration. (hereinafter, the ideal refractive index will be referred to as "designed refractive index of objective lens")

Therefore, the refractive index of the light-transmitting layer needs to be changed to the design refractive index of the objective lens. It is very difficult to change the refractive index of the light-transmitting layer because the refractive index is specific to the material of the light-transmitting layer.

The light transmitting layer may be made of a single material. In this case, the refractive index of the material determines the refractive index of the light-transmitting layer. It is therefore necessary to search for a material having a refractive index that matches the design refractive index of the objective lens. This is not a practical approach. There may be a material that cannot be selected and used due to its other physical properties such as mechanical strength. If the material is chosen to have other desirable characteristics, its refractive index may be different from the design refractive index of the objective lens. If only the thickness of the light-transmitting layer is adjusted, the spherical aberration cannot be minimized.

Disclosure of Invention

An object of the present invention is to provide an optical disc having a light-transmitting layer whose refractive index can be easily adjusted to the design refractive index of an objective lens, thereby greatly reducing spherical aberration.

To achieve the object, there is provided an optical disc including: a substrate, and a recording layer and a light transmitting layer laminated in this order on the substrate, a light beam being applied to the recording layer through the light transmitting layer, thereby recording and/or reproducing data,

wherein the light-transmitting layer is composed of at least two layers of different materials, one on top of the other, the two layers having different refractive indices, at least one of the layers having a refractive index higher than the design refractive index of the objective lens and at least one other layer having a refractive index lower than the design refractive index of the objective lens, the combination of said at least one layer and said other layer adjusting the thickness of said layers so as to reduce the spherical aberration of the objective lens caused by the light-transmitting layer to a value close to a minimum value.

An optical disc according to the present invention includes a substrate, and at least one recording layer and a light-transmitting layer sequentially stacked on the substrate, and a light beam is applied to the recording layer through the light-transmitting layer, thereby recording and/or reproducing data. The light-transmitting layer is composed of at least two laminated material layers of different materials, one on top of the other, having different refractive indices, and is designed to set the spherical aberration caused by an objective lens for focusing a light beam for recording and/or reproducing data to a value close to a minimum value.

In the optical disk thus constituted according to the present invention, the light-transmitting layer is composed of at least two material layers having different refractive indices. The total refractive index of the light-transmitting layer is determined by the refractive index of the material layer. The refractive index of the light-transmitting layer can be made close to the refractive index that the objective lens should have (i.e., the design refractive index of the objective lens) by using a desired material as the material layer or by adjusting the thickness of the material layer. This makes it possible to minimize the spherical aberration.

Therefore, the material of the light-transmitting layer can be selected from a plurality of materials, as compared with the case where only one layer is used.

Other objects and advantages of the present invention will become more apparent from the following detailed description of the embodiments of the present invention, taken in conjunction with the accompanying drawings.

Drawings

FIG. 1 is a cross-sectional view of an optical disc according to the present invention;

FIG. 2 is a graph showing the relationship between the spherical aberration and the refractive index of a light-transmitting layer having a thickness of 100 μm;

fig. 3 is a graph showing the relationship between the thickness and the refractive index when the light-transmitting layer is composed of a single layer;

FIG. 4 is a graph showing the relationship between the thickness of a layer made of a polycarbonate film and the thickness of a layer made of a pressure-sensitive adhesive layer; and

FIG. 5 is a graph showing the relationship between the thickness of a polycarbonate film and the spherical aberration.

Detailed Description

An optical disc of the present invention will be described below with reference to the accompanying drawings.

As shown in fig. 1, an optical disc 1 according to the present invention includes a substrate 2, a reflective layer 3, a recording layer 4, and a light-transmissive layer 5. Layers 3, 4 and 5 are sequentially laminated on substrate 2. The light beam is applied to the recording layer 4 through the light-transmitting layer 5, thereby recording a data signal on the recording layer 4 and reproducing a data signal from the recording layer 4.

The substrate 2 is made of a resin such as polycarbonate or amorphous polyolefin resin. The thickness of the substrate 2 is 0.3mm or more.

The reflective layer 3 is provided on the back surface of the recording layer 4 to reflect a light beam applied thereto. The layer 3 is made of Al or an Al alloy, for example.

The recording layer 4 is a layer on which a data signal is recorded. In this embodiment the layer 4 is made of a phase change material, which changes its reflectivity and thus allows recording of data signals. Can be made of any desired phase change material. Examples of such materials are Ge-Sb-Te, Ag-In-Sb-Te and the like. The recording layer 4 is not necessarily made of a phase change material, and may be made of an organic dye-based recording material, a magneto-optical material, or the like.

The light-transmitting layer 5 protects the reflective layer 3 and the recording layer 4. A light beam emitted from the recording/reproducing apparatus is applied to the light-transmitting layer 5, and a data signal is recorded on and reproduced from the recording layer 4. The light beam thus applied passes through the light-transmitting layer 5 and is focused on the recording layer 4. The light-transmitting layer 5 consists of material layers 6 and 7. The second material layer 7 is located on the first material layer 6. The first material layer 6 and the second material layer 7 have different refractive indices. The refractive index of the combined material layers 6 and 7 thus determines the refractive index of the light-transmitting layer 5. The refractive index of the transparent layer 5 is a value close to the value that minimizes spherical aberration.

For example, the light transmissive layer has an optimum refractive index value or a designed refractive index based on the design of an objective lens provided in the recording/reproducing apparatus, which minimizes spherical aberration.

As indicated before, the light-transmitting layer 5 of the optical disc according to the invention consists of two layers. One of the two layers, i.e., the first material layer 6 or the second material layer 7, is made of a material (high-refractive-index material) having a higher refractive index than the other material (low-refractive-index material) layer. The overall light transmitting layer 5 thus exhibits a refractive index lying between the refractive indices of the first and second material layers 6 and 7. If the refractive index and thickness of the material layers 6 and 7 are adjusted, the refractive index of the layer 5 can be set to a value close to the design refractive index.

The design refractive index, high refractive index material, and low refractive index material will be explained below. As shown in fig. 2, the spherical aberration varies with the change in the refractive index of the light-transmitting layer 5. The spherical aberration is minimized at one particular refractive index, in this case 1.6. This refractive index is referred to as the "design refractive index".

Any material having a refractive index higher than the design refractive index that falls in the region a shown in fig. 2 is referred to as a "high refractive index material". Any material having a refractive index lower than the design refractive index that falls within the region B shown in fig. 2 is referred to as a "low refractive index material".

As mentioned above, the light transmitting layer 5 is a two-layer assembly, consisting of a first material layer 6 and a second material layer 7. Two layers of material having different refractive indices combine to provide one layer exhibiting a particular refractive index. The refractive index of the light-transmitting layer 5 thus determined is close to a value for minimizing spherical aberration. Thus, the light-transmitting layer 5 of the optical disc 1 may have a refractive index that reduces the spherical aberration to a minimum value. Since the refractive index of the light-transmitting layer 5 can be well matched to the design of the lens incorporated in the recording/reproducing apparatus, the light beam applied through the light-transmitting layer 5 undergoes a spherical aberration but is small.

In the optical disk 1 of the present invention, the first material layer 6 is made of a low refractive index material, and the second material layer 7 is made of a high refractive index material. For example, UV resin, pressure sensitive adhesive, or the like may be used as the low refractive index material. Polycarbonate films and the like can be used as the high refractive index material, for example.

In the optical disk 1 of the present invention, the thicknesses of the first and second material layers 6 and 7 are adjusted to have an optimum thickness ratio for further reducing the spherical aberration. Stated differently, it is desirable that the first and second material layers 6 and 7 have an optimal thickness to minimize spherical aberration caused by the light-transmitting layer 5.

The optimum thickness of the first material layer 6 and the optimum thickness of the second material layer 7 will be explained below with reference to some examples.

The first example is an optical disc 1 comprising a light-transmitting layer 5, the light-transmitting layer 5 having a thickness of 100 μm and having a refractive index of 1.60. The first material layer 6 is made of a pressure sensitive adhesive (trade name: DVD8310, manufactured by Nitto Denko co., ltd., refractive index of 1.482). The second material layer 7 is made of a polycarbonate (trade name: C-1400, manufactured by Teijin co., ltd., refractive index of 1.615) film. The inventors have studied to find the optimum thickness of each of the first and second layers 6 and 7. Note that the lens provided in the recording/reproducing apparatus has an NA of 0.85.

First, the present inventors determined the relationship between the refractive index and the thickness of the light-transmitting layer of the optical disc by performing calculation using an optical design calculation program (CODE V; u.s.optical ReaserchAssociate). It should be noted that the light-transmitting layer of the optical disc is composed of a single layer.

It is assumed that spherical aberration is eliminated (reduced to 0) if the refractive index, i.e., the reference refractive index, is 1.6 and the light-transmitting layer 5 is 100 μm thick. Then, if the refractive index of the light-transmitting layer 5 is changed, the light-transmitting layer 5 must have a thickness that minimizes spherical aberration deviating from 100 μm, as can be seen from fig. 3. As shown in fig. 2, the spherical aberration at the minimum point is not zero. The larger the difference between the refractive index and the design refractive index, the more pronounced the spherical aberration. In fig. 3, the abscissa represents the refractive index, and the ordinate represents the thickness that the light-transmitting layer must have to minimize the spherical aberration. In fig. 2, the abscissa represents the refractive index and the ordinate represents the minimum spherical aberration.

Next, the present inventors calculated the light-transmitting layer 5 composed of the first and second material layers 6 and 7 using an optical design calculation program. The purpose of the calculations is to determine how thin the first material layer 6 should be in order to minimize the spherical aberration when the thickness of the second material layer 7 varies. Note that the reference refractive index (design refractive index) is set to 1.6, and the thickness of the light-transmitting layer 5 is 100 μm.

The second material layer 7 is made of a polycarbonate film having a refractive index of 1.615 and a thickness in the range of 50 μm to 100 μm. Therefore, it is only necessary to change the thickness of the first material layer 6 made of a pressure-sensitive adhesive having a refractive index of 1.842, as shown in fig. 4, so that the total thickness of the light-transmitting layer 5 is not 100 μm. In fig. 4, the abscissa represents the polycarbonate film (i.e., the second material layer 7), and the ordinate represents the thickness of the pressure-sensitive adhesive layer (i.e., the first material layer 6).

Fig. 5 shows a graph of the relationship between the thickness of the polycarbonate film and the spherical aberration.

The present inventors found that when the thicknesses of the second material layer 7 made of a polycarbonate film and the first material layer 6 made of a pressure-sensitive adhesive are 90 μm and 10 μm, respectively, the spherical aberration of the light-transmitting layer 5 is reduced to zero. In fig. 5, the abscissa represents the thickness of the polycarbonate film (i.e., the second material layer), and the ordinate represents the spherical aberration.

That is, the spherical aberration can be greatly reduced by changing the thickness of the second material layer 7 made of a high refractive index material having a refractive index exceeding 1.6 and by combining the second material layer 7 with the first material layer 6 made of a low refractive index material having a refractive index smaller than 1.6. The spherical aberration can be eliminated (or reduced to zero) by adjusting the thicknesses of the first material layer 6 and the second material layer 7.

As described above, the optical disk 1 of the present invention has the light-transmitting layer 5 composed of two layers, i.e., the first and second material layers 6 and 7 having different refractive indices. The refractive index of these material layers 6 and 7 determines the refractive index of the light-transmitting layer 5. Thus, the light-transmitting layer 5 of the optical disc 1 may have a refractive index close to that which reduces the spherical aberration to a minimum. This means that the light-transmitting layer 5 has a refractive index which corresponds to the design value of the lens incorporated in the recording/reproducing device. This reduces the spherical aberration of light applied to the recording layer 4 through the light-transmitting layer 5.

In the optical disc 1 according to the present invention, the refractive index of the light-transmitting layer 5 is defined by two materials having different refractive indices and constituting the layer 5. In other words, the refractive index of the light-transmitting layer 5 is determined by the refractive indices of the two materials. Therefore, the material of the light-transmitting layer 5 can be selected from a variety of materials than in the case where the layer 5 is made of only one layer.

In addition, in the optical disc 1 of the present invention, the thicknesses of the first and second material layers 6 and 7 are adjusted to achieve an optimum ratio of the thicknesses. This helps to reduce the spherical aberration of light applied to the recording layer 4 through the light-transmitting layer 5. Therefore, the optical disc 1 according to the present invention can achieve a sufficient reduction in spherical aberration even if the light-transmitting layer 5 has a thickness deviating from the design value.

In the above embodiments, the light-transmitting layer is composed of two layers of different materials having different refractive indices. Needless to say, the light-transmitting layer may be composed of three or more layers stacked on each other. In this case, the layer may also be made of a low refractive index material and a high refractive index material, whereby spherical aberration at the light transmitting layer can be reduced. In the case where the light-transmitting layer is composed of three or more layers, the thicknesses of the layers to be adhered to each other may also be adjusted so as to minimize the spherical aberration of the entire light-transmitting layer.

The above embodiment is an optical disc 1 having a recording layer 4 made of a phase-change material. However, the present invention can be applied to various optical discs in which the recording layer is made of a non-phase-change material, i.e., optical discs in which the reflective layer on each is provided directly on the substrate.

The optical disk according to the present invention has a light-transmitting layer made of two or more layers of materials having different refractive indices laminated on each other. The refractive indices of these layers define the refractive index of the entire light transmitting layer. The refractive index of the light transmitting layer is set to a value that will minimize spherical aberration. I.e., the light transmitting layer has a refractive index close to minimizing spherical aberration. Therefore, the light-transmitting layer may have a refractive index in accordance with a design value of a lens provided in the recording/reproducing apparatus. This minimizes the spherical aberration of light applied to the recording layer through the light transmitting layer.

In the optical disc according to the present invention, the refractive index of the light-transmitting layer is determined by the structure layer made of the low refractive index material and the high refractive index material. I.e. combining two materials of different refractive indices to define the refractive index of the entire light transmitting layer. Thus, the light-transmitting layer according to the present invention can be made of a variety of materials selected from a wide variety of materials, as compared to the case where the layer 5 is made of only one layer.

Claims (2)

1. An optical disc, comprising: a substrate, and a recording layer and a light transmitting layer laminated in this order on the substrate, a light beam being applied to the recording layer through the light transmitting layer, thereby recording and/or reproducing data,

the objective lens is characterized in that the light-transmitting layer is composed of at least two layers of different materials, one layer is arranged above the other layer, the two material layers have different refractive indexes, at least one material layer has a refractive index higher than the design refractive index of the objective lens, at least another material layer has a refractive index lower than the design refractive index of the objective lens, the at least one material layer and the another material layer are combined, and the thickness of the material layers is adjusted, so that the spherical aberration of the objective lens caused by the light-transmitting layer is reduced to a value close to the minimum value.

2. The optical disc as set forth in claim 1, wherein, of the material layers constituting the light-transmitting layer, a material layer having a refractive index higher than a design refractive index of the objective lens is located on a side to which the light beam is applied.