AU2007200467A1 - High-density dual-layer optical disc - Google Patents

Description

P/00o 01 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: High-density dual-layer optical disc The following statement is a full description of this invention, including the best method of performing it known to us:

DESCRIPTION

HIGH-DENSITY DUAL-LAYER OPTICAL DISC S1. TECHNICAL FIELD The present invention relates to a high-density dual-layer optical disc having first and second recording layers, which are both positioned to one side of a central plane bisecting the thickness of the disc, and close to a disc surface.

2. BACKGROUND ART Fig. 1 shows the structure of a normal DVD(Digital Versatile Disc). As shown in Fig. 1, the DVD, which is denoted by the reference numeral 10, has a diameter of 120 mm and a thickness of 1.2 mm, and is formed with a center hole having a diameter of 15 mm, and a clamping region having a diameter of 44 mm and adapted to be clamped by a turntable-and clamper included in an optical disc apparatus.

The DVD 10 has a recording layer, in which data is recorded in a pit pattern. The recording layer of the DVD 10 is positioned at a depth of about 0.6 mm from a disc surface facing an objective lens 1 of an optical pickup device included in the optical disc apparatus. The objective lens 1 of the optical pickup device for the DVD 10 has a numerical aperture NA equal to 0.6.

Fig. 2 shows the structure of a high-density single layer DVD. As shown in Fig. 2, the high-density single layer DVD, which is denoted by the reference numeral 20, has a diameter of 120 mm and a thickness of 1.2 mm, and is formed with a center hole having a diameter of 15 mm, and a clamping region having a diameter of 44 mm and adapted to be clamped by a turntable and clamper included in an optical disc apparatus. The high-density single layer DVD 20 has a data recording layer, which is positioned at a depth of about 0.1 mm from a disc surface facing an objective lens 2 of an optical pickup device included in the optical disc apparatus.

The objective lens 2 of the optical pickup device for the high-density single layer DVD 20 has a numerical aperture NA equal to 0.85, which is a relatively large value in comparison with that of the objective lens 1 for the DVD 10. The objective lens 2 of the optical pickup device adopts a short wave laser beam having a wavelength shorter than that used in the DVD 10 for the reproduction or recording of high-density data.

That is, for the reproduction or recording of high-density data, the DVD 10 uses a laser beam having a wavelength of 650 nm, whereas the high-density single layer DVD 20 uses a laser beam having a wavelength of 405 nm.

By emitting the short wave laser beam and achieving an increase in the numerical aperture of the objective lens, especially in a state of arranging the objective lens 2 of the optical pickup device close to the recording layer of the highdensity single layer DVD 20, it is possible to form a small beam spot on a pit of high data density by intensively focusing the laser beam, and to minimize the length of a transparent layer of the short wave laser beam. As a result, the variation of the laser beam's properties and the occurrence of aberration can be minimized.

In recent years many companies have developed high-density dual-layer optical discs, for example, a high-density dual-layer DVD or high-density dual-layer blu-ray disc(hereafter referred to 004928179 as an "high-density dual-layer as substitutes for the high-density single layer DVD. The high-density dual-layer optical disc can record and store a large quantity of video and audio data, having about twice the capacity of the high-density single layer DVD, for a long time.

In the case of the high-density dual-layer optical disc as stated above, however, there is no way to effectively restrict a wave front error, which is inevitably generated all over the optical disc due to a spherical aberration produced by a variation in the substrate thickness from the light incidence surface of a transparent substrate to respective first and second recording layers and also due to a coma aberration produced by the tilt of the objective lens included in the optical pickup device. Therefore, a solution to this wave front error is urgently required in the field of the high-density dual-layer optical disc.

3. DISCLOSURE OF INVENTION In a first aspect of the present invention there is provided a high-density recording medium, comprising: a light incident surface of the recording medium and an opposite surface of the light incident surface; a substrate including a transparent layer; a first recording layer being located closed to the light incident surface; and a second recording layer being located between the first recording layer and the opposite surface, wherein the minimum thickness from the light incident surface to the first recording layer and the maximum thickness from the light incident surface to the second recording layer depend on a refractive index of the )04928179 transparent layer and the minimum and maximum thickness have larger values in case that the refractive index of the transparent layer is 1.7 than the refractive index of the transparent layer is 1.6, and distance between the first and second recording layers is a value of more than 18.5Vim.

In a second aspect of the present invention there is provided an apparatus for recording or reproducing to or from an optical recording medium, comprising: an optical pickup recording or reproducing a data to or from the optical recording medium, a controller controlling the pickup to record or reproduce data to or from the first and second recording layer of the optical recording medium; wherein recording medium includes a light incident surface of the recording medium, an opposite surface of the light incident surface; a substrate including a transparent layer; a first recording layer being located closed to the light incident surface; and a second recording layer being located between the first recording layer and the opposite surface, further wherein the minimum thickness from the light incident surface to the first recording layer and the maximum thickness from the light incident surface to the second recording layer depend on a refractive index of the transparent layer and the minimum and maximum thickness have larger values in case that the refractive index of the transparent layer is 1.7 than the refractive index of the transparent layer is 1.6, and distance between the first and second recording layers is a value of more than 18.5pm.

004928179 p In a third aspect of the present invention there is provided c-i high-density recording medium, comprising: 1a light incident surface of the recording medium and an opposite surface of the light incident surface; a substrate including a transparent layer; a first recording layer being located closed to the light incident surface; and CI a second recording layer being located between the first Crecording layer and the opposite surface, wherein the first thickness from the light incident surface to the first recording layer is no less than a first value and the second thickness from the light incident surface to the second recording layer is no more than a second value, the first value and the second values depend on a refractive index of the transparent layer, the first value and the second values have a larger value in case that the refractive index of the transparent layer is 1.7 than the refractive index of the transparent layer is 1.6, and distance between the first and second recording layers is a value of more than 18.5[m.

It is an object of the present invention to provide a new high-density dual-layer optical disc having a first and a second recording layers, the optical disc being configured to minimize the generation of a wave front error due to the substrate thickness from a light incidence surface of the transparent substrate to the respective first and second recording layers.

An Example of the high-density dual-layer optical disc is a high-density dual-layer DVD or high-density dual-layer blu-disc.

It is an object of the present invention to provide a new high-density dual-layer optical disc having first and second recording layers, the optical disc being configured to minimize a wave front error generated all over the optical disc due to a 304928179 spherical aberration produced by a variation in the substrate thickness from the light incidence surface of a transparent substrate(i.e. a cover layer) to respectiVe first and second recording layers and also due to a coma aberration produced by the tilt of an objective lens included in an optical pickup device.

In accordance with the present invention, the above and other objects can be accomplished by the provision of a highdensity dual-layer optical disc having a first and a second recording layers positioned to one side of a central plane bisecting the thickness of the disc, and close to a disc surface, a first substrate thickness from a light incidence surface of a transparent substrate to the first recording layer corresponding to a value obtained by subtracting half a distance between the first and the second recording layers from a substrate thickness from a light incidence surface of a transparent substrate to a recording layer in a high-density single layer optical disc, and a second substrate thickness from the light incidence surface of the transparent substrate to the second recording layer corresponding to a value obtained by adding half the distance between the first and second recording layers to the substrate thickness from the light incidence surface of the transparent substrate to the recording layer in the high-density single layer optical disc.

In accordance with the present invention, the above and other objects can be accomplished by the provision of a highdensity dual-layer optical disc having first and second recording layers positioned to one side of a central plane bisecting the thickness of the disc, and close to a light incidence surface, a first substrate thickness from the light incidence surface of the transparent substrate to the first recording layer having a value of more than 70 pi at the minimum, a second substrate thickness from the light incidence surface of the transparent substrate to the second recording layer having a value of less than 108 9n at the maximum, and a distance between the first and second recording layers having a value within a range of 19 jm ±5 ml.

Preferably, thel substrate thickness from the light incidence surface of the transparent substrate to the recording layer in the high-density single layer optical disc may be 0.1 mm.

The distance between the first and the second recording layers may be 0.02 mm. The first and the second substrate.thickness may be 0.09 mm and 0.11 mm, respectively.

Preferably, the first substrate thickness and second substrate thickness may be variably set to an extent that a refractive index n of the transparent substrate is in a range of 1.45 to 1.70. Where the refractive index n of the transparent substrate is equal to 1.60, the first substrate thickness and second substrate thickness may be set at 79.5 pm ±5 pm, and 98.5 pm ±5 pm, respectively.

4. BRIEF DESCRIPTION OF DRAWINGS The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: Fig. 1 shows the structure of a normal DVD; Fig. 2 shows the structure of a general high-density single layer DVD; Fig. 3 shows the example structure of a high-density duallayer optical disc to explain the present invention; Fig. 4 is a graph for comparing a variation in wave front error caused from a spherical aberration with a variation in the substrate thickness from the light incidence surface of a transparent substrate to recording layers in the high-density dual-layer optical disc; Fig. 5 shows the structure of a high-density dual-layer optical disc in accordance with the present invention; SFigs. 6A to 6C are graphs for comparing a variation in wave front error caused from the tilt of an objective lens with a variation in the substrate thickness from the light incidence surface of a transparent substrate to recording layers in the high-density dual-layer optical disc; Fig. 7 is a graph showing the range of the substrate thickness from the light incidence surface of a transparent substrate to first and second recording layers applicable to the high-density dual-layer optical disc in accordance with the present invention; and Fig. 8 shows the structure of a high-density dual-layer optical disc in accordance with an embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION Prior to describing a new high-density dual-layer optical disc in accordance with the present invention, a general highdensity dual-layer DVD or high-density dual-layer BD will be firstly described.

The general high-density dual-layer DVD, which is denoted by the reference numeral 30, has a diameter of 120 mm and a thickness of 1.2 mm, and is formed with a center hole having a diameter of 15 mm, and a clamping region having a diameter of 44 mm and adapted to be clamped by a turntable and clamper included in an optical disc apparatus. The high-density dual-layer DVD comprises a first recording layer, which is formed on the basis of a recording layer of a general high-density single layer DVD, and a second recording layer spaced apart from the first recording layer by a distance of 0.02 mm. In detail, as shown in Fig. 3, the first recording layer of the high-density dual-layer DVD 30 is positioned at a depth of 0.1 mm from a disc surface facing to an objective. lens 2 of an optical pickup device included in the optical disc apparatus, and the second recording layer is positioned at a depth of 0.12 mm from the disc surface.

The objective lens 2 of the optical pickup device for the high-density dual-layer optical disc has a numerical aperture NA equal to 0.85, and adopts a laser beam 4 having a wavelength of 405 nm for the reproduction or recording of high-density data in the first and second recording layers, in the same.manner as the high-density single layer DVD Where the optical pickup device adopting the numerical aperture of 0.85 and the wavelength of 405 nm is used to reproduce or record data in the recording layers, a defocusing margin due to the substrate thickness from the light incidence surface of a transparent substrate to the recording layers is reduced considerably according to the following equation 1.

DFM Eq. (1) At Where, wavelength, NA numerical aperture, and A t: a variation in the substrate thickness from the light incidence surface of a transparent substrate to recording layers.

It should be noted that an increase in the numerical aperture of the objective lens and a decrease in the wavelength result in a significant reduction of the defocusing margin due to a variation in the substrate thickness from the light incidence surface of a- transparent substrate to the recording layers, in comparison with that of the general: DVD. This significant reduction of the defocusing margin ultimately acts to increase system noise.

Meanwhile, in case that first recording layer is formed in a position of 0.1mm from the substrate and second recording layer is formed in a position of 0.08mm from the substrate, that arrangement has more guarantee DFM(De-Focusing Margin) than the case 0.1mm of the first recording layer and 0.12mm of the second recording layer.

Therefore, it is desirable that the second recording layer has thickness less than the first recording layer in view of the

DFM.

That is, the second recording layer is located within the thickness of 0.1mm.

Also, in addition to the DFM, a spherical aberration, comma aberration, and those WFE must be considered when considering the thickness of respective layer.

At first, When it is assumed that the substrate thickness from the light incidence surface of a transparent substrate to a first recording layer is 0.1 mm and the wave front error of a beam spot formed on the recording layer is zero, the wave front error varies with the. substrate thickness from the light incidence surface of the transparent substrate to the second recording layer as shown in the graph of Fig. 4. For example, where the substrate thickness from the light incidence surface of the transparent substrate to the second recording layer is 0.08 mm or 0.12 mm, the wave front error has a value of about 0.18 Xrms.

In general, total aberration shall have a value below than 0.07 k rms in order to not generate an error of large amount in a optical system. In experimental, it is shown that pickup system is no problem if total aberration of pickup has a value below than 0.075 Xrms in an actual system.

Now, hereinafter this invention. will be considered as a status which is below 0.075 Xrms.

As shown in Fig. 4, in case that the thickness to second recording layer from substrate is 0.08mm or 0.12mm, this value considerably exceeds a maximum value of 0.075 Xrms acceptable in the actual system.

As stated above, when the substrate thickness from the light incidence surface of the transparent substrate to the respective first and second recording layers are set at 0.1 mm and 0.12 mm, respectively, or set at 0.1rmm and 0.08 mm, respectively, the wave front error is about 0.18 rms unacceptable in the actual system.

Meanwhile, there are several solutions to compensate for the wave front error as stated above.. That is, by finely regulating the position of a collimator lens 3 included in the optical disc apparatus, or by additionally installing a liquid crystal device and the like to the optical disc apparatus, the wave front error is reduced to about 0.045 Xrms when the substrate thickness from the light incidence surface of the transparent substrate to the second recording layer is 0.08 mm or 0.12 mm.

Fig. 5 shows the structure of a high-density dual-layer optical disc in accordance with the present invention. As shown in .Fig. 5, the high density dual-layer optical disc,* which is denoted by the reference numeral 40, has first and second recording layers.

The first substrate thickness 'tl' from the light incidence surface of a transparent substrate to the first recording layer corresponds to a value obtained by subtracting half the distance between the first and second recording layers from the substrate thickness from the light incidence surface of a transparent substrate to a recording layer in a general high-density single layer optical disc.

The second substrate thickness 't2' from the light incidence surface of the transparent substrate to the second recording layer.

corresponds to a value obtained by adding half the distance between the first and second recording layers to the substrate thickness from the light incidence surface of the transparent substrate to the recording layer in the general high-density single layer optical disc.

That is, the high-density dual-layer DVD or high-density dual-layer BD of the present invention has a diameter of 120 mm and a thickness of 1.2 mm, and is formed with a center hole having a diameter of 15 mm, and* a clamping region having a diameter of 44 mm and adapted to be clamped by a turntable and clamper included in an optical disc apparatus. The high-density dual-layer DVD 40 of the present invention is provided with the first recording layer positioned. at a depth of 0.09 mm from a disc surface facing an objective lens 2 of an optical pickup device included in the optical disc apparatus, and the second recording layer positioned at a depth of 0.11 mm from the disc surface facing the objective lens 2 of the optical pickup device.

Therefore, under the condition as stated above referring to Fig. 4, when the first and second substrate thickness from the light incidence surface of the transparent substrate to the respective first and second recording layers is set at 0.09 mm and 0.11 mm, respectively, the wave front error is only about 0.08 Xrms, close to a maximum value of 0.075 Xrms acceptable in the actual system. Furthermore, by virtue of finely regulating the position of a. collimator lens 3 and the installation of the additional compensation liquid crystal device, the wave front error is reduced to about 0.025 Xrms. In this way, the generation of the wave front error due to the substrate thickness from the light incidence surface of the transparent substrate to the recording layers can be effectively restricted.

Figs. 6A to 6C are graphs for comparing a variation in wave front error caused from the tilt of the objective lens with a variation in the substrate thickness from the light incidence.

surface of the transparent substrate to recording layers in the high-density dual-layer optical.disc. Referring to Figs. 6A to 6C, a spherical aberration produced by a variation in the substrate thickness from the light incidence surface of a transparent substrate(i.e. a cover layer) to recording layers, in a no-tilt d) state of an objective lens included in an.optical pickup device, S defines a line (D respectively shown in Figs. 6A to 6C.- A coma aberration, produced in a state that the objective Slens of the optical pickup device has..a tilt angle of less than 5.0.60°, defines a line respectively shown in Figs. 6A to 6C. A:

O

O wave front error generated all over the optical disc caused from sp the spherical. aberration and coma aberration defines a line. O 'respectively shown in Figs. 6A to 6C.

S In Figs. 6A to 6C, the line CD is drawn by applying the graph shown: in Fig. 4, and is obtained from the following equation 2.

t(n 2 -1 2n 2

)NA

3 a Eq. (2) where is Thickness, is Refractive ration, NA is .Numerical Aperture of objective lens, 'a '.is amount of Tilt.

In general, a general optical system considers, a .maximum amount of tilt as 0.6, therefore the comma aberration is. applied -to the.equation based on the value.

That is, the value of the wave front error is calculated according to the following equation 3.

2/ Eq (31 Eq. (3) Where, spherical aberration produced by a variation in the substrate thickness from the light incidence surface of a transparent substrate to recording layers under a no-tilt state of an objective lens, coma aberration produced under a tilt .angle of less than 0.6at the maximum, and wave front error generated all .over the optical disc due to the spherical 11 a) aberration and coma aberration.

c-i Therefore, as shown in Fig. 6A, the substrate thickness from the light incidence surface of the transparent substrate to respective first and second recording layers has to be set within 5 a range of about 70 m to 108 fm, in order to satisfy a maximum wave front error value of 0.075 X rms acceptable in an actual csystem.

This result is a value obtained from a consideration of Refractive Index, which means a refractivity of optical disc.

In particular, this result is based on the refractive index of 1.60.

Further, as shown in Fig. 6B, the substrate thickness from the light incidence surface of the transparent substrate to respective first and second recording layers has to be set within a range of about 68.5 gm to 106.5 nl, in order to satisfy a maximum wave front error value of 0.075 Xrms acceptable in an actual system.

Therefore, as shown in Fig. 6C, the substrate thickness from the light incidence surface of the transparent substrate to respective first and second recording layers has to be set within a range of about 71.4 pm to 11.5 n, in order to satisfy a maximum wave front error value of 0.075 Xrms acceptable .in an actual system.

This will be described in detail below.

Fig. 7 is a graph showing the range of the substrate thickness* from the light incidence surface of a .transparent substrate to first and second recording layers applicable to a high-density dual-layer optical. disc in accordance with the present invention. As shown in Figs. 6A to 6C, the substrate thickness from the light incidence surface of the transparent substrate to the recording layers is variably set in accordance with a refractive index of the transparent substrate.

For example, where the refractive index n of the transparent substrate is equal to 1.60, the substrate thickness from the light incidence surface of the transparent substrate to the recording layers has to be in a range of about 70 pm to 108 9m for satisfying the maximum wave front error value of 0.075 Xrms.

In case that the same condition is considered to other refractive index, where the refractive index n of the transparent substrate is equal to 1.45, as shown in Fig. 5B, the substrate thickness from the light incidence surface of the transparent substrate to the recording layers has to be in a range of about 68.5 Jim to 106.5 ml for satisfying the maximum wave front error value of 0.075 Xrms.

In addition, where the refractive index n of the transparent substrate is equal to 1.70, as shown in Fig. 5C, the substrate thickness from the light incidence surface of the transparent substrate to the recording layers has to be in a range of about 110.5 pm to 71.4 p for satisfying the maximum wave front .error value of 0.075 rms.

In conclusion, the substrate thickness from the light incidence surface of the transparent substrate to the first recording layer is in a range of about 108 pma 2.5 (or m at the maximum, and the substrate thickness from the light incidence surface of the transparent substrate to -the second recording layer is in a range of. about 70 jan 1.4(or jm at the minimum.

Therefore, referring to Fig. 8 showing the structure of the high-density dual-layer optical disc in accordance with an embodiment of the present invention, the substrate thickness from the light incidence surface of the transparent substrate to the first recording layer is set at a value of 70 pa at the minimum, 13 the substrate thickness from the light incidence surface of the transparent substrate to the second recording layer is set at a value of 108 mI at the maximum, and also a distance between the first and second recording layers is set in a range of 19 pm ±5 pm.

Now, this will be described in more detail below.

The first and second recording layer can be divided into an average of those values, that is, 89pm 70+108/2) as a boundary, for example, when the first recording layer has the minimum value of 70pI, the second recording layer must have .89jm that is a boundary of value, and when the second recording layer has 108lm, the first recording layer must have 89m that is a boundary of value.

Therefore, the distance between the first recording layer and second recording layer can be set to 19pm. And, if it is considered by manufacturing error margin, it can be set to the value of 19pm±5pm, which is acceptable in current system.

Though the thickness can be considered to a value broader than above value, it desirable that its error margin is ±5pm when the technology for manufacturing the recording substrate is to be considered. Therefore, a average value between respective layers is most stable if a distance between respective layers is searched for a representative as 191m1. That is, the average value is 79.5/m and 98.5n1 respectively if we calculate the average of respective ranges of layers. According to this result, the substrate thickness from the light incidence surface of the transparent substrate to the respective first and second recording layers are set at 79.5 pm ±5 pm and 98.5 pm ±5 pm, respectively.

Therefore, as shown in Fig. 8, where the refractive index n of the transparent substrate is equal to 1.60, the substrate thickness from the light incidence surface of the transparent substrate to the respective first and second recording layers are set at 79.5 gm and 98.5: mi, respectively, and the distance between the first and second recording layers is set in a range of 19 n gm. In this case, according to the permitted distance limit of ±5 pm,. the substrate thickness from the light incidence surface of the transparent substrate to the respective first and second recording layers are set at 79.5 um ±5 im and 98.5 mi ±5 um, respectively.

According to the configuration of the high-density duallayer optical disc, it is possible to effectively restrict the wave front error generated all over the optical disc due to the spherical aberration produced by a variation in the substrate thickness from the light incidence surface of the transparent substrate to the respective first and second recording layers and also due to the coma aberration. produced. by the tilt of the objective lens.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

As apparent from the above description, the present invention provides a high-density dual-layer optical disc for minimizing a wave front error generated all over the optical disc due to a spherical aberration produced by a variation in the substrate thickness -from a light incidence surface of a transparent substrate to respective first and second recording layers and also due to a coma aberration produced by the tilt of an objective lens, and for enabling the more accurate recording or reproduction of signals onto or from the optical disc.

Claims (3)

1. A high-density recording medium, comprising: a light incident surface of the recording medium and an opposite surface of the light incident surface; a substrate including a transparent layer; a first recording layer being located closed to the light incident surface; and a second recording layer being located between the first recording layer and the opposite surface, wherein the minimum thickness from the light incident surface to the first recording layer and the maximum thickness from the light incident surface to the second recording layer depend on a refractive index of the transparent layer and the minimum and maximum thickness have larger values in case that the refractive index of the transparent layer is 1.7 than the refractive index of the transparent layer is 1.6, and distance between the first and second recording layers is a value of more than 18.5[m.

2. A high-density optical disc of claim i, wherein distance between the first and second recording layers is less than

19.5pm 3. A high-density optical disc of claim 1, wherein the minimum and maximum thickness have the minimum values when the refractive index of the transparent layer is 1.45 4. A high-density optical disc of claim 3, wherein the minimum thickness has a value of more than 68.5pm. A high-density optical disc of claim 4, wherein the maximum thickness has a value of less than 110.5pm 6. An apparatus for recording or reproducing to or from an optical recording medium, comprising: 01928179 an optical pickup recording or reproducing a data to or from the optical recording medium, a controller controlling the pickup to record or reproduce data to or from the first and second recording layer of the optical recording medium; wherein recording medium includes a light incident surface of the recording medium, an opposite surface of the light incident surface; a substrate including a transparent layer; a first recording layer being located closed to the light incident surface; and a second recording layer being located between the first recording layer and the opposite surface, further wherein the minimum thickness from the light incident surface to the first recording layer and the maximum thickness from the light incident surface to the second recording layer depend on a refractive index of the transparent layer and the minimum and maximum thickness have larger values in case that the refractive index of the transparent layer is 1.7 than the refractive index of the transparent layer is 1.6, and distance between the first and second recording layers is a value of more than 18.5 Im. 7. An apparatus of claim 6, wherein a distance between the first and second recording layers is a value of less than 19.5 lm 8. An apparatus of claim 6, wherein the minimum and maximum thickness have the minimum values when the refractive index of the transparent layer is 1.45 9. An apparatus of claim 8, wherein the minimum thickness has a value of more than 68.5Lm. 10. An apparatus of claim 9, wherein the maximum thickness has 004928119 a value of less than 110.5tm. D 11. A high-density recording medium, comprising: a light incident surface of the recording medium and an O opposite surface of the light incident surface; a substrate including a transparent layer; \O a first recording layer being located closed to the light incident surface; and a second recording layer being located between the first recording layer and the opposite surface, wherein the first thickness from the light incident surface to the first recording layer is no less than a first value and the second thickness from the light incident surface to the second recording layer is no more than a second value, the first value and the second values depend on a refractive index of the transparent layer, the first value and the second values have a larger value in case that the refractive index of the transparent layer is 1.7 than the refractive index of the transparent layer is 1.6, and distance between the first and second recording layers is a value of more than 18.5im.