AU2007200467B2 - High-density dual-layer optical disc - Google Patents

Description

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 1. TECHNICAL FIELD The present invention relates to a high-density dual-layer 5 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 10 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 15 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 20 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 25 is denoted by the reference numeral 20, has a diameter of 120 mm

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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 camper included in an optical disc apparatus. The high-density 5 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 10 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 15 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. 20 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 high density single layer DVD 20, it is possible to form a small beam 25 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. 30 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 2 as an "high-density dual-layer BD") , 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 5 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 0 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 5 error is urgently required in the field of the high-density dual-layer optical disc. It is not admitted that any of the information in this specification is common general knowledge, or that the person skilled in the art could be reasonably expected to have 0 ascertained, understood, regarded it as relevant or combined it in anyway at the priority date. 3. DISCLOSURE OF INVENTION In a first aspect of the present invention there is provided a high-density recording medium, comprising: 25 a light incidence surface of the recording medium and an opposite surface of the light incidence surface; a first recording layer being located close to the light incident surface; 3 a second recording layer being located between the first recording layer and the opposite surface; and a transparent substrate formed between the light incident surface and the first recording layer, and including a cover 5 layer, 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 cover layer and the minimum 0 and maximum thickness have larger values in the case where the refractive index of the cover layer is 1.7 than the case where the refractive index of the cover layer is 1.6, and distance between the first and second recording layers is a value of more than 14pm. 15 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 20 pickup to record or reproduce data to or from the first and second recording 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; 25 a first recording layer being located close to the light incident surface; a second recording layer being located between the first recording layer and the opposite surface; and 4 a transparent substrate formed between the light incident surface and the first recording layer, and including a cover layer, further wherein the minimum thickness from the light incident surface to 5 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 cover layer and the minimum and maximum thickness have larger values in the case where the refractive index of the cover is 1.7 than the case where the 0 refractive index of the cover layer is 1.6, and distance between the first and second recording layers is value of more than 14pm. In a third aspect of the present invention there is provided a high-density recording medium, comprising: 5 a light incident surface of the recording medium and an opposite surface of the light incident surface; a first recording layer being located close to the light incident surface; a second recording layer being located between the first 20 recording layer and the opposite surface; and a transparent substrate formed between the light incident surface and the first recording layer, and including a cover layer, wherein the first thickness from the light incident surface to 25 the first recording layer is no less than a first value and the second thickness from the light incident surface to the second layer is no more than a second value, the first value and the second values depending on a refractive index of the cover 5 layer, the first value and the second value having a larger value in the case where the refractive index of the cover is 1.7 than in the case where the refractive index of the cover layer is 1.6, and distance between the first and second 5 recording layers being a value of more than 14pm. It is an object of at least a preferred embodiment 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 the generation of a 0 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. 5 It is an object of at least a preferred embodiment 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 spherical 0 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. 25 The above and other objects may be accomplished by the provision of a high-density 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 disc surface, a first substrate thickness from a light incidence 30 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 5a 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 5 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. 0 The above and other objects may be accomplished by the provision of a high-density 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 5 incidence surface of the transparent substrate to the first recording layer having a value of more than 70pm 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 108pm at the maximum, and a distance between .0 the first and second recording layers having a value within a range of 19pm + 5pm. Preferably, the 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 0.1mm. The 25 distance between the first and the second recording layers may be 0.02mm. The first and the second substrate thickness may be 0.09mm and 0.11mm, respectively. Preferably, the first substrate thickness and second substrate thickness may be variably set to an extent that a 30 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 5b second substrate thickness may be set at 79.5pm + 5pm, and 98.5pm + 5pm, respectively. As used herein, except where the context requires otherwise the term 'comprise' and variations of the term, such as 5 'comprising', 'comprises' and 'comprised', are not intended to exclude other additives, components, integers or steps. 4. BRIEF DISCUSSION OF DRAWINGS The above and other objects, features and other advantages of the present invention will be more clearly understood from 0 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; 5 Fig. 3 shows the example structure of a high-density dual layer 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 20 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; 5c Figs. 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 5 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 10 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. 5. MODES FOR CARRYING OUT THE INVENTION 15 Prior to describing a new high-density dual-layer optical disc in accordance with the present invention, a general high density dual-layer DVD or high-density dual-layer BD will be firstly described. The general high-density dual-layer DVD, which is denoted by 20 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 camper included in an optical disc apparatus. The high-density dual-layer DVD 30 25 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 30 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 6 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 5 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 20. Where the optical pickup device adopting the numerical 10 aperture of 0.85 and the wavelength of 405 nrm 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. 5 DFM= ...................................... Eq-.(l) 15(NA)'At Where, X 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 20 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 25 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 30 arrangement has more guarantee DFM(De-Focusing Margin) than the 7 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 5 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 10 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 15 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 20 wave front error has a value of about 0.18 X rms. In general, total aberration shall have a value below than 0.07 krms 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 25 0.075 X rms in an actual system. Now, hereinafter this invention. will be considered as a status which is below 0.075 X rms. As shown in Fig. 4, in case that the thickness to second recording layer from substrate is 0.08mm or 0.12mm, this value 30 considerably exceeds a maximum value of 0.075 Xrms acceptable in the actual system. 8 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'.1mm and 0.08 mm, respectively, the wave 5 front error is about 0.18 X 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 10 and the like to the optical disc apparatus, the wave front error is reduced to about 0.045 X rms 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 15 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 20 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. 25 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 30 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 9 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 5 camper 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 10 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 15 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 20 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. 25 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, 30 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 10 state of an objective lens included in an optical pickup device, defines.a line (])respectively shown in Figs. 6A to 6C. A coma aberration, produced in a state that the objective lens of the optical pickup device has a tilt angle of less than 5. 0.60, defines a line (respectively shown i6A to 6C. A wave front error generated all over the optical disc caused from the spherical aberration and coma aberration defines a line. -respectively shown in Figs. 6A to 6C. In Figs. 6A to 6C, the line 01 is drawn by applying the graph 10 shown. in Fig. 4, andGIs obtained from the following equation 2. = t(n 2 -1 / 2n 2 NAIa ............ ................ Eq. (2) where 't' is Thickness, In' is Refractive ration, NA is - Numerical Aperture of objective lens, 'a 'is amount of Tilt. In general, a general optical system considers, a .maximum 15 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. ... .......................................... Eq. (3) 20 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 0.6*at the maximum, and : wave front 'error 25 generated all -over the optical disc due to the spherical aberration and coma aberration. 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 Ium to 108 pm, in order to satisfy a maximum wave front error value .of 0.075 Xrms acceptable in an actual system. This result is a value obtained from a consideration of Refractive Index, which means a refractivity of optical disc. 10 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 15 a range of about 68.5 pm to 106.5 ym, in order to satisfy a maximum wave front error value of 0.075 X rms acceptable in an actual system. Therefore, as shown in Fig. 6C, the substrate thickness from the light incidence surface of the transparent substrate to 20 respective first and second recording layers has to be set within a range of about 71.4 pm to 11.5 pM, 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. 25 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 30 thickness from the light incidence surface of the transparent substrate to the recording layers is variably set in accordance 12 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 5 layers has to be in a range of about 70 ym to 108 9M for satisfying the maximum wave front error value of 0.075 Arms. 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 10 thickness from the light incidence surface of the transparent substrate to the recording layers has to be in a range of about 68.5 nh to 106.5 pm for satisfying the maximum wave* front error value of 0.075 A rms. In addition, where the refractive index n of the transparent 15 substrate *is equal to 1.70, as shown in Fig. SC, the substrate thickness frorn 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 pm for satisfying the maximum wave front. error value of 0.075 A rms. 20 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 ym + 2.5 (or -1.5) .jm at the maximum, and the substrate thickness from the light incidence surface of the transparent substrate to - the second 25 recording layer is in'a range of. about 70 jam + 1.4(or -1.5) jn 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. 30 the light incidence surface of the transparent substrate to the first recording layer is set at a value of 70 pm 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 ImI at the maximum, and also a distance between the first and second recording layers is set in a range of 19 ym ±5 pm. 5 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 70pm, the second recording . layer must have .-89pm that is a 10 boundary of value, and when the second recording layer has 108pm, 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 15 considered by manufacturing error margin, it can be set to the value of 19#m±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 20 considered. Therefore, a average value between respective layers is most stable if a distance between respective layers is searched for a representative as 191mi. That is, the average value is 79.5pmn and 98.5pm respectively if we calculate the average of respective ranges of layers. According to this result, the substrate 25 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 30 thickness from the light incidence surface of the transparent 14 substrate to the respective first and second recording layers are set at 79.5 pm and 98.5 fm, respectively, and the distance between the first and second recording layers is set in a range of 19 jm ±5 pm. In this case, according to the permitted distance limit of 5 ±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 ym ±5 pm and 98.5 pm ±5 pm, respectively. According to the configuration of the high-density dual 10 layer 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 15 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 20 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 25 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 30 or reproduction of signals onto or from the optical disc. 15

Claims (11)

1. A high-density recording medium, comprising: a light incidence surface of the recording medium and an opposite surface of the light incidence surface; 5 a first recording layer being located close to the light incident surface; a second recording layer being located between the first recording layer and the opposite surface; and a transparent substrate formed between the light incident 0 surface and the first recording layer, and including a cover layer, 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 5 depend on a refractive index of the cover layer and the minimum and maximum thickness have larger values in the case where the refractive index of the cover layer is 1.7 than the case where the refractive index of the cover layer is 1.6, and distance between the first and second recording layers is a value of more 20 than 14pm.

2. The high-density optical disc of claim 1, wherein distance between the first and second recording layers is less than 24pm.

3. The high-density optical disc of claim 1, wherein 25 the minimum and maximum thickness have the smallest values when the refractive index of the cover layer is 1.45. 16

4. The high-density optical disc of claim 3, wherein the minimum thickness has a value of more than 68.5pm.

5. The high-density optical disc of claim 4, wherein the maximum thickness has a value of less than 110.5pm. 5

6. 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 0 second recording 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 first recording layer being located close to the light 5 incident surface; a second recording layer being located between the first recording layer and the opposite surface; and a transparent substrate formed between the light incident surface and the first recording layer, and including a cover 20 layer, 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 cover layer and the minimum and 25 maximum thickness have larger values in the case where the refractive index of the cover is 1.7 than the case where the refractive index of the cover layer is 1.6, and distance 17 between the first and second recording layers is value of more than 14pm.

7. An apparatus of claim 6, wherein a distance between the first and second recording layer is a value of less than 5 24pm.

8. An apparatus of claim 6, wherein the minimum and maximum thickness have the smallest values when the refractive index of the cover layer is 1.45.

9. An apparatus of claim 8, wherein the minimum 0 thickness has a value of more than 68.5pm.

10. An apparatus of claim 9, wherein the maximum thickness has a value of less than 110.5pm.

11. A high-density recording medium, comprising: a light incident surface of the recording medium and an 5 opposite surface of the light incident surface; a first recording layer being located close to the light incident surface; a second recording layer being located between the first recording layer and the opposite surface; and 20 a transparent substrate formed between the light incident surface and the first recording layer, and including a cover layer, wherein the first thickness from the light incident surface to the first recording layer is no less than a first value and the 25 second thickness from the light incident surface to the second 18 layer is no more than a second value, the first value and the second values depending on a refractive index of the cover layer, the first value and the second value having a larger value in the case where the refractive index of the cover is 5 1.7 than in the case where the refractive index of the cover layer is 1.6, and distance between the first and second recording layers being a value of more than 14pm. 19