Abstract:
A method for forming an optical disk including, 
 
providing a substrate and an information recording layer on the substrate, and covering the information recording layer with a cover layer having light transmission properties such that a thickness t of the cover layer is determined based on a predetermined function f(n), which depends on a refractive index n of the cover layer, and the function f(n) is,  
           f   ⁡     (   n   )       =           A   1     ×     n   3         (       n   2     -   1     )       ×       (       n   2     +     A   2       )       (       n   2     +     A   3       )       ⁢           ⁢     (   mm   )         ,       
        in which, A 1 =0.049245, A 2 =−0.579135, and A 3 =0.012842, and constants t 1  and t 2  determine a range of the thickness t of the cover layer so that f(n)−t 1 &lt;t&lt;f(n)++t 2.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part and claims the benefit of priority under 35 U.S.C. §120 from U.S. application Ser. No 11/171,289 and U.S. application Ser. No. 10/107,404 filed Mar. 28, 2002 and claims the benefit of priority under 35 U.S.C. § 119 from Japanese Patent Application No. 2001-372939, filed Dec. 6, 2001. The entire contents of these applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an improvement in an optical disk enabling high-density recording of information and a recording/reproducing apparatus of the optical disk.  
         [0004]     2. Description of the Related Art  
         [0005]     As well known, in recent years, a digital versatile disk (DVD) has been practically used as an optical disk in which information can be recorded with a high density. The DVD has an information recording capacity which is as much as 4.7 giga bytes (GB) on one surface layer.  
         [0006]     There are prepared a plurality of types of DVD. Example of the DVD include a DVD-read only memory (DVD-ROM) for exclusive use in reproduction, a DVD-random access memory (DVD-RAM) in which data can be rewritten, and the like. passed through the transparent substrate and focused onto the information recording layer, and the information is written or read.  
         [0007]     For a numerical aperture (NA) of an objective lens for focusing the laser beam, a reference value is 0.6. A refractive index n of the transparent substrate is designated in a range of 1.45 to 1.65 with respect to a laser beam having a wavelength of 650 nm.  
         [0008]     Therefore, a substrate material which satisfies the aforementioned condition is selected for the transparent substrate. Polycarbonate is generally used as the substrate material. In this case, the refractive index n of the transparent substrate is 1.58.  
         [0009]     As described above, the reference value of thickness of the transparent substrate constituting the DVD is 0.6 mm. Additionally, as an actual problem, generation of a dispersion in the thickness of the transparent substrate during manufacturing cannot be avoided.  
         [0010]     However, for an optics for guiding the laser beam to the DVD, the thickness of the transparent substrate of the DVD is designed as the reference value of 0.6 mm. Therefore, when the thickness of the transparent substrate deviates from 0.6 mm, aberration is generated.  
         [0011]     It is necessary to suppress the aberration of the optics to a constant value or less. When the aberration increases, a diameter of a beam spot focused on the information recording layer of the DVD increases, and recording/reproducing of the information is adversely affected.  
         [0012]     The aberration of the optics caused by the dispersion of the thickness of the transparent substrate is determined by both a deviation from the designed value of the thickness of the transparent substrate and a deviation from the designed value of the refractive index of the transparent substrate.  
         [0013]     In the DVD, the range of the thickness allowed for the transparent substrate is defined as a two-dimensional range with the refractive index n of the transparent substrate so that the aberration of the optics caused by the dispersion of the thickness of the transparent substrate is suppressed to a constant value or less.  
         [0014]     The two-dimensional range is disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 8-273199. In the publication, for a range of 1.45 to 1.65 of the refractive index n of the transparent substrate, an error is set to ±0.02 mm with respect to the reference value of the thickness of the transparent substrate.  
         [0015]     That is, in a graph indicating the refractive index n on the abscissa and the thickness of the transparent substrate on the ordinate, when the refractive index n is smaller than a lens load specification, the two-dimensional range is defined not in a simple rectangular form, but in a form deviating in a direction for increasing the thickness of the transparent substrate.  
         [0016]     At present, a technical development of the DVD has been advanced for a further high-density recording. The diameter of the beam spot focused on the information recording layer of the DVD is proportional to the wavelength of the laser beam, and inversely proportional to the numerical aperture NA which indicates a focus angle of the objective lens.  
         [0017]     Therefore, the numerical aperture NA of the objective lens needs to be increased in order to reduce the diameter of the beam spot focused on the information recording layer of the DVD so that the information recording density of the DVD is enhanced.  
         [0018]     On the other hand, when the optical disk tilts with respect to an incidence direction of the laser beam because of influences of warp, and the like, an optical path of the laser beam passed through the transparent substrate becomes asymmetrical, and a frame aberration is generated in the laser beam focused on the information recording layer.  
         [0019]     A frame aberration amount is approximately proportional to the cube of the value of the numerical aperture NA of the objective lens. Therefore, when the numerical aperture NA is increased for the high-density recording, a very large frame aberration is caused by a slight property change of the optical disk.  
         [0020]     The frame aberration amount is proportional to the thickness of a light transmission layer of the optical disk. Therefore, for a next-generation optical disk for achieving the high-density recording, the reduction of the thickness of the light transmission layer is under consideration in order to secure a margin for inclination of the optical disk.  
         [0021]     Examples of the numerical aperture NA of the objective lens and the thickness of the light transmission layer in the next-generation optical disk include a numerical aperture NA of the objective lens=0.85, and a reference value of the thickness of the light transmission layer=0.1 mm.  
         [0022]     Moreover, regarding a thickness error of the light transmission layer in the optical disk which achieves the recording with a density higher than that of the existing DVD, a proposal is disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 2000-11454.  
         [0023]     That is, assuming that the wavelength of the laser beam is λ, the publication proposes that a thickness unevenness Δt of the light transmission layer of the optical disk be set to:
 
|Δ t|≦ 5.26 λ/NAˆ 4.
 
         [0024]     However, when the thickness unevenness Δt of the light transmission layer of the optical disk is defined by the above equation, a range is derived using a thickness unevenness standard in a compact disk (CD) system as a reference, and is inappropriate from the following two viewpoints.  
         [0025]     (1) The thickness range of the light transmission layer should originally be defined with an absolute value in consideration of the refractive index n of the light transmission layer as in the existing DVD. On the other hand, only the thickness range of the light transmission layer is defined by the above equation.  
         [0026]     (2) Introduction of a mechanism for correcting a spherical surface aberration generated with the enhanced NA is considered in the next-generation optical disk. It is not appropriate any more to define the thickness range of the light transmission layer using the CD system as the reference.  
         [0027]     The above (1) is a condition required when the whole drive system of the optical disk is considered. That is, in the optical disk drive, the thickness and refractive index of the optical disk to be recorded/reproduced are principally determined as the specifications of the objective lens constituting the optics without any exception.  
         [0028]     Therefore, the optical disk mounted on the optical disk drive is naturally requested to have the same values of the thickness and refractive index of the light transmission layer as specified values of the optical disk drive.  
         [0029]     However, when the optical disk is manufactured, or when the material of the light transmission layer for use in the optical disk is selected by a maker, the thickness and refractive index of the light transmission layer of the optical disk mounted on the optical disk drive inevitably have constant margins. A problem lies in an allowed degree of the margin.  
         [0030]     It is adequate to use the load specified value of the objective lens of the optical disk drive as the reference and to define the allowable margin as the range of the thickness and refractive index of the light transmission layer which gives a constant wave front aberration by the deviation from the reference value.  
         [0031]     On the other hand, the above equation simply defines the range of the thickness of the light transmission layer. When the optics specifications of the optical disk drive are considered, a concept of the refractive index required to be simultaneously considered is naturally lacking. It has to be said that the definition is inappropriate as a system including the optical disk and disk drive.  
         [0032]     In the DVD, the constant range of the thickness and refractive index of the transparent substrate (light transmission layer) is already determined as the standard under the aforementioned idea. However, with the enhancement of NA, needless to say, the standard cannot be valid in the next-generation optical disk whose specified value of the thickness of the light transmission layer largely differs.  
         [0033]     The above (2) is a characteristic prerequisite of the next-generation optical disk. In the next-generation optical disk, the increase of the frame aberration with the enhancement of NA can be compensated for by the reduction of the light transmission layer as described above.  
         [0034]     On the other hand, with the enhancement of NA, the increase of the spherical surface aberration accompanying the thickness error of the light transmission layer also becomes remarkable. This is because the spherical surface aberration is approximately proportional to a value obtained by multiplying the NA four times.  
         [0035]     Even when the thickness of the light transmission layer is reduced, the spherical surface aberration cannot be compensated. Therefore, in the next-generation optical disk, the introduction of the mechanism for correcting the spherical surface aberration is newly considered.  
         [0036]     The correction mechanism of the spherical surface aberration has not been introduced in the conventional DVD or CD system. Here is a large difference from the conventional system. Therefore, when the conventional DVD and CD system are considered as the standard in defining the error range of the thickness of the light transmission layer, an inappropriate result is obviously obtained.  
         [0037]     That is, on the assumption that the next-generation optical disk includes the correction mechanism of the spherical surface aberration, it is necessary to select the reference value of the wave front aberration different from that of the conventional system and to define the range of the thickness of the light transmission layer.  
         [0038]     For example, it is assumed in the conventional system that the reference value of the wave front aberration is 0.04 λrms or less. However, in the next-generation optical disk, when an effect obtained by the mechanism for correcting the spherical surface aberration is considered, the reference value of the wave front aberration can be moderated down to 0.10 λrms.  
         [0039]     As described above, when the correction mechanism of the spherical surface aberration is introduced in the drive system of the next-generation optical disk, and when the conventional system is considered as the standard, an erroneous result is brought about. The above equation uses the CD system as the standard, but the standard is not appropriate in the next-generation optical disk.  
         [0040]     In the specifications of the next-generation optical disk, for example, with λ=405 nm, NA=0.85, the above equation designates the thickness unevenness range of |Δt|≦4.08 μm. This is an unreasonably narrow range, when the presence of the correction mechanism of the spherical surface aberration is considered. Therefore, the yield in the production of the optical disk is excessively decreased.  
       BRIEF SUMMARY OF THE INVENTION  
       [0041]     The present invention has been developed in consideration with the above circumstances, and an object thereof is to provide an optical disk and recording/reproducing apparatus of the optical disk in which effective ranges of thickness of a light transmission layer and refractive index of the light transmission layer are defined, for example, in a next-generation optical disk and which are suitable for a high-density recording.  
         [0042]     According to one aspect of the present invention, there is provided an optical disk comprising:  
         [0043]     an information recording layer formed on a substrate; and  
         [0044]     a cover layer which has a light transmission property and with which the information recording layer is covered,  
         [0045]     wherein ranges of a thickness and a refractive index of the cover layer are set such that an aberration generated by deviations from specified values of the thickness and the refractive index of the cover layer is within a constant allowable value.  
         [0046]     According to another aspect of the present invention, there is provided a recording/reproducing apparatus of an optical disk constituted by covering an information recording layer formed on a substrate with a cover layer having a light transmission property and setting ranges of a thickness and a refractive index of the cover layer so that an aberration generated by deviations from specified values of the thickness and the refractive index of the cover layer is within a constant allowable value, the apparatus comprising:  
         [0047]     a correcting section which corrects a spherical surface aberration in the cover layer during irradiation of the optical disk with a laser beam from the cover layer. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0048]      FIG. 1  is a side sectional view showing a detailed structure of an optical disk according to a first embodiment of the present invention;  
         [0049]      FIG. 2  is a block diagram showing an optical disk drive for performing recording/reproducing with respect to the optical disk in the first embodiment;  
         [0050]      FIG. 3  is an explanatory view of an rms value of aberration of the optical disk using a relation between a refractive index and a thickness of a cover layer as a parameter;  
         [0051]      FIG. 4  is an explanatory view of ranges of the refractive index and thickness of the cover layer of the optical disk, when an allowable aberration in the first embodiment is 0.10 λrms;  
         [0052]      FIG. 5  is an explanatory view of the ranges of the refractive index and thickness of the cover layer of the optical disk, when the allowable aberration in the first embodiment is 0.04 λrms;  
         [0053]      FIG. 6  is an explanatory view of the ranges of the refractive index and thickness of the cover layer of the optical disk in a known example;  
         [0054]      FIG. 7  is an explanatory view showing another example of the ranges of the refractive index and thickness of the cover layer of the optical disk in the known example;  
         [0055]      FIG. 8  is an explanatory view of the ranges of the refractive index and thickness of the cover layer of the optical disk according to a second embodiment of the present invention;  
         [0056]      FIG. 9  is a side sectional view showing a detailed structure of the optical disk according to a third embodiment of the present invention;  
         [0057]      FIG. 10  is an explanatory view of the ranges of the refractive index and thickness of the cover layer of the optical disk in the third embodiment; and  
         [0058]      FIG. 11  is an explanatory view of the ranges of the refractive index and thickness of the cover layer of the optical disk according to a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0059]     A first embodiment of the present invention will be described hereinafter in detail with reference to the drawing. First,  FIG. 1  shows a structure of an optical disk  11  described in the first embodiment in a section.  
         [0060]     In the optical disk  11 , for example, an information recording layer  13  including a phase change recording film is formed on a substrate  12  formed of polycarbonate. When the optical disk  11  is a disk for exclusive use in reproduction, the information recording layer  13  by a metal reflective film is formed instead of the phase change recording film.  
         [0061]     A cover layer (light transmission layer)  14  having a thickness t is formed on the information recording layer  13 . The cover layer  14  is a sheet which is formed of a plastic material and which has a thickness t. The cover layer  14  is bonded onto the information recording layer  13  via an adhesive mass, ultraviolet cured resin, or the like.  
         [0062]      FIG. 2  shows an optical disk drive which performs recording/reproducing with respect to the optical disk  11 . The optical disk drive uses a short-wavelength semiconductor laser beam source  20  as a light source. An outgoing light  100  from the semiconductor laser beam source  20  is a light having a violet wavelength band, for example, in a range of 395 nm to 415 nm.  
         [0063]     The outgoing light  100  from the semiconductor laser beam source  20  is transformed to a parallel light by a collimator lens  21 , successively passed through a polarized beam splitter  22 , λ/4 plate  23 , and relay lens system  24 , and incident upon an objective lens  25 . Thereby, the light is passed through the cover layer  14  of the optical disk  11 , and focused onto the information recording layer  13 .  
         [0064]     A reflected light  101  from the information recording layer  13  of the optical disk  11  is again passed through the cover layer  14  of the optical disk  11 , successively transmitted through the objective lens  25 , relay lens system  24 , and λ/4 plate  23 , reflected by the polarized beam splitter  22  substantially at a right angle, transmitted through a photodetect lens system  26 , and incident upon a photodetector  27 .  
         [0065]     The photodetector  27  has a light receiving section divided into a plurality of regions, and outputs a current corresponding to a light intensity from each light receiving region. The current outputted from each light receiving region of the photodetector  27  is converted to a voltage by an I/V amplifier (not shown), and supplied to an operation circuit  30 .  
         [0066]     The operation circuit  30  calculates/processes a voltage signal corresponding to each light receiving region of the photodetector  27 , and thereby generates a high frequency (HF) signal, a focus error signal, a tracking error signal, a control signal of the relay lens system  24 , and the like.  
         [0067]     The focus error signal and tracking error signal are supplied to a driver  29  via a servo driver  31 , and thereby controlled in a focus direction and tracking direction of the objective lens  25 .  
         [0068]     The relay lens system  24  is constituted of a bottom lens  24   a  and top lens  24   b.  The top lens  24   b  is controlled in an optical axis direction, when the control signal of the relay lens system  24  outputted from the operation circuit  30  is supplied to a driver  28  via the servo driver  31 .  
         [0069]     The relay lens system  24  is designed such that the laser beam is incident substantially as a parallel light upon the objective lens  25 , when the thickness of the cover layer  14  is of a defined value (e.g., 100 μm).  
         [0070]     When the thickness of the cover layer  14  deviates from the defined value, a spherical surface aberration occurs because of a thickness error of the cover layer  14 . In this case, the shape of a beam spot focused on the information recording layer  13  of the optical disk  11  is strained. Therefore, it becomes difficult to perform a stable and accurate recording/reproducing.  
         [0071]     On the other hand, when the incident light upon the objective lens  25  is converted to a convergent or divergent light, the spherical surface aberration is generated. Moreover, when the top lens  24   b  of the relay lens system  24  is moved in the optical axis direction, the incident light upon the objective lens  25  can be converted to the convergent or divergent light.  
         [0072]     Therefore, when the top lens  24   b  of the relay lens system  24  is moved in the optical axis direction in accordance with a thickness error amount of the cover layer  14 , and the incident light upon the objective lens  25  is converted to the convergent or divergent light, the spherical surface aberration generated by the thickness error of the cover layer  14  can be corrected.  
         [0073]     Concretely, when the thickness of the cover layer  14  is larger than a defined value, the top lens  24   b  of the relay lens system  24  is moved in the optical axis direction in accordance with the thickness error amount of the cover layer  14  so that the incident light upon the objective lens  25  is converted to the divergent light.  
         [0074]     Conversely, when the thickness of the cover layer  14  is smaller than the defined value, the top lens  24   b  of the relay lens system  24  is moved in the optical axis direction in accordance with the thickness error amount of the cover layer  14  so that the incident light upon the objective lens  25  is converted to the convergent light.  
         [0075]     In the optical disk drive which performs the recording/reproducing with respect to the next-generation optical disk in this manner, there is assumed to be provided means for correcting the spherical surface aberration generated by the thickness error from the defined value of the cover layer  14  of the optical disk  11 .  
         [0076]     This has not been considered in the CD or the DVD as the conventional optical disk system. Therefore, when the conventional definition is applied as such during the manufacturing of the next-generation optical disk, an erroneous result is produced.  
         [0077]     The optical disk  11  described in the first embodiment is assumed to have the ranges of the thickness error and refractive index of the cover layer  14  in which the recording/reproducing in the optical disk drive including the correction means of the spherical surface aberration is taken into consideration.  
         [0078]     For the specifications of the optical disk drive which performs the recording/reproducing with respect to the next-generation disk, for example, the wavelength of the laser beam is 405 nm, and the numerical aperture NA of the objective lens  25  is 0.85. The use of the specifications is considered.  
         [0079]     Moreover, it is assumed that the cover layer  14  has a refractive index of 1.622 and thickness of 100 μm, and the objective lens  25  has the aberration completely corrected and is ideal with respect to the optical disk  11  having such lens load.  
         [0080]     The optical disk  11  having various and different values of the refractive index n and thickness t of the cover layer  14  is used with respect to the objective lens  25 , and the rms value of the generated wave front aberration is obtained. The results are shown in  FIG. 3 .  
         [0081]      FIG. 3  shows the refractive index n of the cover layer  14  on the abscissa, the thickness t of the cover layer  14  on the ordinate, and the rms values of the aberration in respective points on a coordinate plane with contour lines. The contour line has a graduation which corresponds to 2/100 of the wavelength (=405 nm) of the laser beam. In  FIG. 3 , a point shown by a double circle indicates a point of the reference specifications, that is, the load specifications of the objective lens  25 , and the aberration turns to 0 in this point.  
         [0082]     As seen from the result, when the optical disk  11  having various and different values of the refractive index n and thickness t of the cover layer  14  is used and, for example, when the refractive index n is larger than the lens load specified value in order to set a residual aberration amount to be constant, it is better to set the thickness t of the cover layer  14  to be slightly larger than the specified value.  
         [0083]     Therefore, the cover layer  14  of the optical disk  11  in the next-generation DVD needs to be defined such that the error allowable range of the thickness t of the cover layer  14  is changed as the absolute value in accordance with the refractive index n of the cover layer  14 .  
         [0084]     The ranges of the refractive index n and thickness t of the optical disk  11  described in the first embodiment are shown in  FIG. 4 . This shows the following region.  
               Refractive   ⁢           ⁢   index   ⁢     :     ⁢           ⁢   1.47     ≤   n   ≤   1.67           (   1   )                   Cover   ⁢           ⁢   layer   ⁢           ⁢   thickness   ⁢     :     ⁢           ⁢     f   ⁡     (   n   )         -     t   ⁢           ⁢   1       ≤   t   ≤       f   ⁡     (   n   )       +     t   ⁢           ⁢   2               (   2   )                   f   ⁡     (   n   )       =           A   1     ×     n   3         (       n   2     -   1     )       ×       (       n   2     -     A   2       )       (       n   2     -     A   3       )       ⁢           ⁢     (   mm   )         ⁢     
     ⁢         A   1     =   0.049245     ,     
     ⁢       A   2     =     -   0.579135       ,     
     ⁢       A   3     =   0.012842       ⁢     
     ⁢     t   ⁢           ⁢   1     ,       t   ⁢           ⁢   2     =     10   ⁢           ⁢     (   µm   )                 (   3   )             
 
         [0085]     This substantially agrees with a range in which the aberration in the contour line diagram shown in  FIG. 3  is 0.1 λrms or less. When the optical disk  11  having this range is defined, the aberration by the deviation from the specified values of the thickness t and refractive index n of the cover layer  14  can substantially be kept to 0.1 λrms or less.  
         [0086]     The contour lines indicating the wave front aberration amounts in  FIG. 3  are arranged substantially in parallel in a coordinate direction, and a curve is obtained by giving a constant offset to the equation (3). Therefore, when the allowable value of the aberration is determined, the ranges of the thickness t and refractive index n of the cover layer  14  can be determined by the above equations (1) to (3).  
         [0087]     In this case, when t 1 , t 2  are changed in accordance with the allowable value of the aberration, the range of the thickness t of the cover layer  14  may be adjusted. For example, when the allowable aberration is 0.04 λrms, t 1 , t 2 =4 μm is set in the above equations, and an appropriate range can be designated (see  FIG. 5 ).  
         [0088]     On the other hand, the range of the refractive index n is determined by the material of the cover layer  14  and the wavelength of the laser beam. Therefore, a range including an effective material of the cover layer  14  of the optical disk  11  can be defined.  
         [0089]     In this case, with n=about 1.47 to 1.67, the effective materials such as polycarbonate are used as the cover layer (light transmission layer)  14  of the optical disk  11 , and the refractive index n in the violet wavelength band can be covered.  
         [0090]     In the conventional optical disk drive, the aberration by the thickness and refractive index error of the light transmission layer of the optical disk is limited to 0.02 to 0.03 λrms. Therefore, for example, when the allowable value of the aberration is set to 0.02 λrms, it is necessary to set the error range of the thickness t of the cover layer  14  to t 1 , t 2 =2 μm from  FIG. 3 .  
         [0091]     In this case, it is easily imagined that the manufacturing margin of the optical disk  11  is very small. However, it is assumed in the next-generation optical disk that the correction means of the spherical surface aberration is introduced as described above.  
         [0092]     Therefore, the allowed aberration amount can be enlarged as compared with the conventional optical disk drive. For example, when the allowed aberration amount is set to 0.1 λrms as described above, t 1 , t 2 =10 μm, the manufacturing margin of the optical disk  11  can be set to be large, and the enhancement of the yield can be expected.  
         [0093]     On the other hand, specifications in which the thickness error of the cover layer is set to be constant regardless of the refractive index are considered as in the aforementioned known example. This corresponds to the use of the above equation (3) as a constant (specified value of the cover layer thickness).  
         [0094]     In this case, for example, with t 1 , t 2 =2 μm, a region is obtained as shown in  FIG. 6 . As seen from  FIG. 6 , the region does not agree with a region in which the wave front aberration indicates a constant value or less.  
         [0095]     As a comparative example of the first embodiment, an example of t 1 , t 2 =10 μm is considered, and a region is obtained as shown in  FIG. 7 . Also in this case, the value of the wave front aberration largely changes in a region boundary, and the aberration reaches 0.12 λrms, for example, where n=1.47, t=110 μm.  
         [0096]     Therefore, with t 1 , t 2 =10 μm in the first embodiment, considering that the wave front aberration is suppressed to 0.10 λrms or less in all the regions, a necessity of increasing the allowed aberration amount occurs.  
         [0097]     Conversely, in the conventional example, the allowable range of the thickness error of the cover layer has to be narrowed with respect to a certain allowed aberration amount. That is, the margin in manufacturing the disk is reduced.  
         [0098]     A second embodiment of the present invention will next be described.  FIG. 8  shows the range of the thickness t and refractive index n of the cover layer  14  of the optical disk  11  described in the second embodiment. This range is substantially similar to the range of the optical disk  11  of the first embodiment shown in  FIG. 4 , and the region is shown as a range surrounded with a straight line, not a curve.  
         [0099]     That is, in  FIG. 8 , a plurality of (three in the drawing) points are sampled from a curve which indicates an aberration of 0.10 λrms, and the range is set with the straight lines which connect the sampled points. An effect substantially equal to that of the optical disk  11  of the first embodiment can also be obtained by the range set as described above.  
         [0100]     A third embodiment of the present invention will be described.  FIG. 9  shows a section of an optical disk  51  described in the third embodiment. The optical disk  51  is constituted by forming an information recording layer  53  including, for example, a phase change recording film on a substrate  52  formed, for example, of polycarbonate.  
         [0101]     A transparent intermediate layer  54  is formed on the information recording layer  53 , and another information recording layer  55  is formed on the intermediate layer  54 . The information recording layers  53 ,  55  may be layers for exclusive use in reproduction by metal reflective films, or recordable/reproducible layers. Alternatively, one of the layers may be the layer for exclusive use in reproduction, while the other may be the recordable/ reproducible layer.  
         [0102]     A cover layer (light transmission layer)  56  is formed on the information recording layer  55 . The cover layer  56  is a sheet formed, for example, of a plastic material. The cover layer  56  is bonded onto the information recording layer  55  via a pressure-sensitive adhesive or an ultraviolet cured resin.  
         [0103]     The intermediate layer  54  has a function of optically interrupting leak of information (crosstalk) from one information recording layer  53  or  55 , while the other information recording layer  55  or  53  is reproduced.  
         [0104]     In this sense, an interval between two information recording layers  53 ,  55  may be as large as possible, and the intermediate layer  54  may preferably be thicker. However, in this case, the recording/reproducing optics bears a burden.  
         [0105]     That is, when the thickness of the surface of the cover layer  56  to the center of the intermediate layer  54  is defined as the load of the objective lens  25 , the aberration by the thickness error of half the thickness of the intermediate layer  54  is generated even during the recording/reproducing of either the information recording layer  53  or  55 .  
         [0106]     Therefore, from a viewpoint of the aberration of the recording/reproducing optics, the intermediate layer  54  needs to be preferably thin. That is, the thickness of the intermediate layer  54  is determined by a point of compromise of the crosstalk between the information recording layers  53  and  55  with respect to a tradeoff relation in the aberration of the recording/reproducing optics.  
         [0107]     For example, when the wavelength of the laser beam is set to 405 nm, and the numerical aperture NA of the objective lens  25  is set to 0.85 as the specifications of the optical disk drive for performing the recording/reproducing with respect to the next-generation optical disk, an appropriate thickness of the intermediate layer  54  is in a range of about 20 to 30 μm in consideration of the above tradeoff.  
         [0108]     It is preferable to represent the defined thickness of the light transmission layer of the two-layers type optical disk  51  by a minimum value of the thickness of the cover layer  56  and a maximum value of a total thickness of the cover layer  56 , information recording layer  55  disposed adjacent to the cover layer  56 , and intermediate layer  54 .  
         [0109]     The ranges of the thickness and refractive index of the light transmission layer of the optical disk  51  are shown in  FIG. 10 . Similarly as the aforementioned embodiment, the lens load of the light transmission layer having a refractive index of 1.622 and thickness of 100 μm in the optical disk  51  is assumed.  
         [0110]     In the defined region, the refractive index is 1.47≦n≦1.67, the thickness of the cover layer  56  is f(n)−t 1  or more, the thickness of the cover layer  56 + information recording layer  55 + intermediate layer  54  is f(n)+t 2  or less, t 1 , t 2 =20 μm, and f(n) is represented by the above equation (3). As a result, since the thickness of the intermediate layer  54  is added, the range of the thickness direction is broadened as compared with a one-layer constitution.  
         [0111]     A fourth embodiment of the present invention will be described.  FIG. 11  shows the ranges of the thickness and refractive index of the cover layer  56  of the optical disk  51  described in the fourth embodiment.  
         [0112]     The range is substantially similar to that of the optical disk  51  of the third embodiment shown in  FIG. 10 , and the region is shown as the range surrounded with the straight line, not the curve. Also in this case, a plurality of (three in the drawing) points are sampled from the curve indicating the aberration of 0.20 λrms, and the range is set with the straight line which connects the sampled points.  
         [0113]     An effect substantially equal to that of the optical disk  51  of the third embodiment can be obtained even by the range set in this manner. Moreover, the third and fourth embodiments show two information recording layers  53 ,  55 , but the present invention can, needless to say, be applied to the optical disk having three or more information recording layers.  
         [0114]     Additionally, the present invention is not limited to the aforementioned embodiments, and can variously be implemented within a range which does not depart from the scope.